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A Chandra X-Ray Survey of Optically Selected Close Galaxy Pairs: Unexpectedly Low Occupation of Active Galactic Nuclei

A Chandra X-Ray Survey of Optically Selected Close Galaxy Pairs: Unexpectedly Low Occupation of... High-resolution X-ray observations offer a unique tool for probing the still-elusive connection between galaxy mergers and active galactic nuclei (AGNs). We present an analysis of nuclear X-ray emission in an optically selected sample of 92 close galaxy pairs (with projected separations 20 kpc and line-of-sight velocity offsets −1 <500 km s ) at low redshift (z ¯ ~ 0.07), based on archival Chandra observations. The parent sample of galaxy pairs is constructed without imposing an optical classification of nuclear activity, thus it is largely free of selection effect for or against the presence of an AGN. Nor is this sample biased for or against gas-rich mergers. An X-ray +5% source is detected in 70 of the 184 nuclei, giving a detection rate of 38% , down to a 0.5–8 keV limiting -5% 40 −1 luminosity of 10 erg s . The detected and undetected nuclei show no systematic difference in their host galaxy properties such as galaxy morphology, stellar mass, and stellar velocity dispersion. When potential contamination 41 −1 +3% from star formation is avoided (i.e., L > 10 erg s ), the detection rate becomes 18% (32/184), which 2−10 keV -3% shows no excess compared to the X-ray detection rate of a comparison sample of optically classified single AGNs. +2% The fraction of pairs containing dual AGN is only 2% . Moreover, most nuclei at the smallest projected -2% separations probed by our sample (a few kiloparsecs) have an unexpectedly low apparent X-ray luminosity and Eddington ratio, which cannot be solely explained by circumnuclear obscuration. These findings suggest that close galaxy interaction is not a sufficient condition for triggering a high level of AGN activity. Unified Astronomy Thesaurus concepts: Galaxy nuclei (609); Interacting galaxies (802); Galaxy mergers (608); X- ray active galactic nuclei (2035) Supporting material: machine-readable tables 1. Introduction can significantly grow its mass, preceding the formation of an SMBH binary and their ultimate merger (Merritt & It is a generic prediction of the standard paradigm of Milosavljević 2005). hierarchical structure formation that most galaxies frequently As observational validation of the above scenario, a number interact with other galaxies during their lifetime. When the two of systematic searches for dual AGN candidates have been interacting galaxies are gravitationally bound, their ultimate conducted over the past decade, primarily in the optical band, fate is to merge, eventually forming a more massive galaxy. In thanks to wide-field, homogeneous spectroscopic surveys such the course of galaxy mergers, tidal force and ram pressure act as the Sloan Digital Sky Survey (SDSS). In particular, the to significantly redistribute the stellar and gaseous contents of search for galactic nuclei with double-peaked narrow emission the interacting pair (Toomre & Toomre 1972; Barnes & lines (e.g., [O III]; Wang et al. 2009; Liu et al. 2010) aims at Hernquist 1992). It is theoretically predicted and has been tight AGN pairs (typically 1–10 kpc in separation, but even demonstrated by numerical simulations (e.g., Di Matteo et al. less) that pertain to the late stage of merger, whereas the search 2005) that upon close passages, gravitational torques drive gas for resolved pairs of galactic nuclei both showing the optical inflows to the center of one or both galaxies, potentially emission-line characteristics of Seyfert or Low Ionization triggering nuclear star formation and active galactic nuclei Nuclear Emission-line Region (LINER) covers larger projected (AGNs). A physical consequence of this scenario is the separations up to ∼100 kpc (Liu et al. 2011, hereafter L11). prevalence of AGN pairs in (major) galaxy mergers, which Confirmation of the AGN nature in these optically selected involve two SMBHs with simultaneous active accretion. candidates, however, often require follow-up observations in Specifically, “dual AGNs,” AGN pairs with a separation the X-ray and/or radio bands (Comerford et al. 2011; 10 kpc in projection, are generally expected at the inter- Silverman et al. 2011; Teng et al. 2012; Liu et al. 2013;Fu mediate-to-late stage of mergers (see recent review by De Rosa et al. 2015a, 2015b; Brightman et al. 2018; Gross et al. 2019; et al. 2019). This is a crucial phase during which the SMBH(s) Hou et al. 2019; Foord et al. 2020), which are generally thought to trace immediate radiation from the SMBH (more precisely, Original content from this work may be used under the terms from the accretion disk, corona, and/or jets) and tend to be of the Creative Commons Attribution 4.0 licence. Any further more immune to circumnuclear obscuration. Infrared observa- distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. tions have also played an effective role in revealing dual 1 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. AGNs, especially in gas-rich merging systems, which tend to pre-selection of optical AGN characteristics as applied in Hou select highly obscured AGNs (Satyapal et al. 2014, 2017; et al. (2020), thus allowing for an unbiased view of AGN activity through their nuclear X-ray emission. This paper is Pfeifle et al. 2019). An alternative approach (Koss et al. 2012) structured as follows. Section 2 describes the construction of a starts with a hard X-ray (10 keV) AGN detected in the Swift/ new sample of close galaxy pairs with archival Chandra BAT survey and tries to associate it with another AGN in a observations. Data analysis toward detection and characteriza- companion galaxy within a projected distance of 100 kpc, if it tion of the nuclei are detailed in Section 3. Section 4 presents exists. This approach, however, inevitably introduced selection the results, including the properties and statistics of the X-ray bias toward X-ray-luminous AGNs due to the moderate detected nuclei and a reexamination of the behavior of L as sensitivity of Swift/BAT. Nevertheless, these approaches have 2−10 a function of r . Section 5 summarizes the study and address achieved a certain degree of success, revealing a growing p most significant implications. Throughout this work, we number of AGN pairs and dual AGNs. assume a concordance cosmology with Ω = 0.3, Ω = 0.7, Clearly, having a sizable and unbiased sample of genuine m Λ −1 −1 and H = 70 km s Mpc . Errors are quoted at 1σ confidence dual AGNs is crucial for a thorough understanding of the 0 level unless otherwise stated. causality between galaxy mergers and AGN triggering. Recently, Hou et al. (2020, hereafter H20) carried out a systematic search for X-ray-emitting AGN pairs, using archival 2. The Sample of Close Galaxy Pairs Chandra observations and based on the Liu et al. (2011) sample In this work, we construct a new sample of close galaxy of ∼10 optically selected AGN pairs at low redshift (with a pairs based on the parent sample of galaxy pairs recently median redshift z ~ 0.1). Thanks to the superb angular presented by Feng et al. (2019, hereafter F19). The F19 sample resolution of Chandra, unattainable from any other X-ray itself was extracted from the SDSS DR7 (Abazajian et al. 2009) facility, one can unambiguously resolve and localize the photometric galaxy catalog, with ∼95% of the cataloged putative AGN even in close pairs. More importantly, the typical galaxies having an available spectroscopic redshift, which was sensitivity of Chandra observations used by Hou et al. (2020) is primarily from SDSS and supplemented by LAMOST (Luo sufficient to probe low-luminosity AGNs (i.e., weakly accreting et al. 2015; Shen et al. 2016), GAMA (Baldry et al. 2018), and SMBHs) down to a limiting 2–10 keV X-ray luminosity of 40 −1 other spectroscopic surveys (see detailed description in Feng L ∼ 10 erg s , which is necessary for a complete census 2−10 et al. 2019). A close galaxy pair was selected if the two of nuclear activity. member galaxies have a line-of-sight velocity offset Among 67 pairs of the optically selected AGN candidates −1 Δv < 500 km s and a projected separation r  20 kpc. We with useful Chandra data, Hou et al. (2020) found that 21 pairs also required that each galaxy has only one neighbor galaxy show significant X-ray emission from both nuclei (i.e., with a similar redshift within a projected separation of 100 kpc probable AGN pairs), with an additional 36 pairs having only −1 and a velocity offset of 500 km s , to minimize environmental one of the two nuclei detected. The X-ray detection rate of all effects typical of compact groups or clusters. Contrary to Feng 134 nuclei, 58% ± 7% (1σ Poisson errors), is significantly −1 et al. (2019), who focused on pairs with r > 10h kpc, we higher than that (17% ± 4%) of a comparison sample of star- impose no lower limit on r . However, due to the resolution forming galaxy pairs, classified also based on optical emission- limit of the optical surveys (∼1″), the Feng et al. (2019) sample line ratios. Moreover, interesting trends were revealed for the still suffers from incompleteness for the most closely separated mean X-ray luminosity as a function of the projected pairs (i.e., 1 kpc). separation, r , which is taken as a proxy for the merger phase, We thus have a preliminary list of 3337 close galaxy pairs. A where larger (smaller) r represents the earlier (later) stage of a comparison with the Liu et al. (2011) sample of optically selected merger. First, L increases with decreasing projected 2−10 AGN pairs shows that the two samples have 130 common pairs, separation in AGN pairs at r  20 kpc, suggesting enhanced whereas 3207 pairs are in the Feng et al. (2019) sample but not in SMBH accretion even in early-stage mergers, perhaps related the Liu et al. (2011) sample. This difference partly stems from the to the first pericentric passage of the two galaxies. Second and fact that the Liu et al. (2011) sample, which was primarily based unexpectedly, L decreases (rather than increases) with 2−10 on SDSS DR7 spectroscopic redshifts, suffers from the restriction decreasing r at r  10 kpc, which appears contradicting with p p of SDSS fiber collision and thus is missing closely separated the intuitive expectation that tidal-force-driven gas inflows galaxy pairs. The Feng et al. (2019) sample was exactly designed become more and more prevalent as mergers proceed. Despite to overcome this incompleteness, thereby significantly increasing the small number statistics, Hou et al. (2020) proposed two the number of close galaxy pairs. Moreover, the Liu et al. (2011) physical explanations for this latter behavior: (i) merger- sample required both galaxies in a pair to have a Seyfert or induced gas inflows become so strong that an enhanced central LINER classification based on the optical emission-line diag- concentration of cold gas heavily obscures even the hard (2–10 nostics, whereas the Feng et al. (2019) sample only required a keV) X-rays; (ii) AGN feedback triggered by the first spectroscopic redshift based primarily on stellar continuum, thus pericentric passage acts to expel gas from the nuclear region it, in principle, minimizes the selection bias for or against AGN and consequently suppress or even halt SMBH accretion. The activity in closely interacting galaxies (though see Section 4.4 for latter possibility is of particular interest, potentially offering potential bias for the most luminous AGNs in a few Chandra insight into the still-elusive processes of SMBH feeding and observations), as well as selection bias for or against gas-rich feedback during an indispensable stage of galaxy evolution. mergers. Extending the study of Hou et al. (2020), in this work we use We cross-matched the Feng et al. (2019) sample with the archival Chandra observations to survey the nuclear X-ray Chandra X-ray data archive to select pairs with observations emission from a new sample of close galaxies pairs. These taken with the Advanced CCD Imaging Spectrometer (ACIS) close galaxies pairs are selected from optical spectroscopic and publicly available as of June 2022. Similar to Hou et al. surveys (see Section 2 for details), but they are not subject to a (2020), we requested that both galactic nuclei in a pair fall 2 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Figure 1. Redshift (left panel), r-band absolute magnitude (middle panel), and projected angular separation (right panel) distributions of the close galaxy pairs studied in this work (black solid histogram), in comparison with the close AGN pairs (blue dashed) in Hou et al. (2020). The vertical lines mark the median value of the individual samples. within the ACIS field of view and within 8¢ from the aimpoint, while the close AGN pairs have a similar z = 0.062 to ensure the feasibility of source detection and photometry. and M =-21.4 mag. We further visually inspected the SDSS to filter several spurious galaxy pairs, which are most likely compact star- 3. Data Analysis forming clusters/complexes that mimicked a second galactic 3.1. Chandra Data Preparation nucleus. Our final sample consists of 92 optically and X-ray selected galaxy pairs, which have r ranging from 3.0 to The Chandra/ACIS data were reprocessed following the 19.7 kpc. This small fraction (92/3337) reflects the empirical standard procedure, using CIAO v4.13 with the calibration files rule that on average only a few percent of randomly selected CALDB v4.9.5. Among the 92 galaxy pairs in the current sky targets would fall on a Chandra/ACIS footprint. Basic sample, 78 pairs have only one observation, while the the other information of these galaxy pairs are given in Table 1. 14 pairs have been observed more than one time, for which we Our sample is an extension of the AGN pairs and SFG pairs combined all available observations. studied by Hou et al. (2020). The Hou et al. (2020) AGN pairs, Following the procedures in Hou et al. (2020), for each selected from the parent sample of Liu et al. (2011), cover a observation we produced counts, exposure, and point-spread wider range of projected separations (r < 100 kpc) and have function (PSF) maps on the natal pixel scale of 0 492 in the both nuclei classified as an AGN based on the optical emission- 0.5–2 (S),2–8 (H), and 0.5–8 (F) keV band. The exposure line diagnostics. Hou et al. (2020) also constructed a maps and the PSF maps were weighted by a fiducial incident comparison sample of SFG pairs (i.e., both nuclei having the spectrum, which is an absorbed power-law with a photon index optical emission-line diagnostics of star formation). Consider- of 1.7 (a median value for AGN, see Winter et al. 2009) and 22 −2 ing only the close pairs (i.e., those with r < 20 kpc) in Hou absorption column densities N = 10 cm for the H band 21 −2 et al. (2020), there are 28 AGN pairs and 12 SFG pairs. For and N = 10 cm for the S band. clarity, hereafter we refer to AGN pairs or SFG pairs of Hou For targets with multiple observations, the counts, exposure, et al. (2020) as those pairs with r < 20 kpc only, unless and PSF maps of individual observations were reprojected to a otherwise stated. With our new sample, which presumes no common tangential point after calibrating their relative distinction between optically classified AGN and SFG, the total astrometry, to produce combined images that maximize the number of close galaxy pairs with both Chandra and optical source detection sensitivity. Only the I0, I1, I2, and I3 CCDs spectroscopic observations is now more than doubled. We note for the ACIS-I observations and the S2 and S3 CCDs for the that the new sample includes 17 AGN pairs and 1 SFG pair in ACIS-S observations were included at this step. We have Hou et al. (2020). These pairs are kept in the following examined the light curves of each observation and filtered time analysis, but caution is taken not to double-count them when an intervals contaminated by significant particle flares, if any. The analysis also involves those pairs from Hou et al. (2020). There effective exposure time of each target pair ranged from 1.1 to also existsome pairs that belong to Hou et al. (2020) but are 240.1 ks, with a median value of 13.7 ks. not included in the new sample. This is mainly due to the fact that the Liu et al. (2011) sample did not impose the requirement 3.2. X-Ray Counterparts and Photometry on the absence of a third galaxy within 100 kpc and also included some pairs that are not part of the parent galaxy We followed the procedures detailed in Hou et al. (2020) to sample of Feng et al. (2019). search for X-ray counterparts of the optical nuclei in our close Figure 1 compares the redshift (left panel) and SDSS r-band galaxy pairs. We first performed source detection in the 0.5–2, absolute magnitude (M ; middle panel) distributions of the 2–8, and 0.5–8 keV bands for each galaxy pair using the CIAO current sample with those of the AGN pairs in Hou et al. tool wavdetect, with the 50% enclosed count fraction (ECF) (2020). The current sample has a median redshift z ¯ = 0.067 and a median r-band absolute magnitude M =-21.1 mag, http://cxc.harvard.edu/ciao/ 3 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 1 Information of Close Galaxy Pairs with Chandra Observation Name R.A. Decl. zr log M SFR log M log L Flag p * BH X,lim (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) +3.04 J102700.40+174901.0 156.75167 17.81694 0.0665 3.0 10.9 1.09 7.4 40.17 0 -0.97 +29.44 J102700.56+174900.3 156.75233 17.81675 0.0666 3.0 L 12.86 L 40.17 1 -9.38 +8.31 J085837.53+182221.6 134.65637 18.37267 0.0587 3.3 10.4 3.55 7.5 40.39 1 -2.62 +9.05 J085837.68+182223.4 134.65700 18.37317 0.0589 3.3 11.1 3.46 7.7 40.39 1 -2.88 +2.73 J105842.44+314457.6 164.67683 31.74933 0.0728 4.1 10.0 1.95 6.1 40.71 1 -1.19 +5.33 J105842.58+314459.8 164.67742 31.74994 0.0723 4.1 10.9 2.44 7.5 40.70 1 -1.75 +0.15 J002208.69+002200.5 5.53621 0.36681 0.0710 4.2 11.0 0.02 7.8 40.41 1 -0.02 +0.14 J002208.83+002202.8 5.53679 0.36744 0.0707 4.2 11.2 0.02 8.1 40.44 1 -0.02 +0.49 J133031.75−003611.9 202.63229 −0.60331 0.0542 4.4 8.8 0.35 L 40.44 0 -0.21 +6.69 J133032.00−003613.5 202.63333 −0.60375 0.0542 4.4 10.7 3.98 7.4 40.44 1 -2.57 +0.71 J141447.15−000013.3 213.69646 −0.00369 0.0475 4.9 10.5 0.23 6.9 40.04 1 -0.22 +4.94 J141447.48−000011.3 213.69783 −0.00314 0.0474 4.9 10.2 6.0 40.03 1 3.54 -2.12 J235654.30−101605.4 359.22628 −10.26817 0.0739 4.9 LL L 41.10 1 +0.23 J235654.49−101607.4 359.22708 −10.26875 0.0732 4.9 9.3 2.47 L 41.03 0 -0.17 +0.01 J091931.14+333852.1 139.87977 33.64782 0.0237 5.1 8.5 0.13 L 40.70 0 -0.03 J091930.30+333854.4 139.87628 33.64845 0.0237 5.1 LL L 40.78 0 J093529.56+033923.1 143.87320 3.65644 0.0463 5.4 LL L 41.52 0 +0.11 J093529.77+033918.1 143.87408 3.65505 0.0464 5.4 10.0 0.81 L 41.55 0 -0.13 +0.21 J122814.15+442711.7 187.05896 44.45325 0.0233 5.5 10.7 0.06 6.9 41.06 1 -0.06 J122815.23+442711.3 187.06348 44.45314 0.0229 5.5 LL L 41.01 1 J112648.50+351503.2 171.70212 35.25089 0.0322 5.9 LL L 40.02 1 +0.18 J112648.65+351454.2 171.70274 35.24839 0.0321 5.9 10.0 2.03 6.7 40.01 1 -0.39 +0.13 J090025.61+390349.2 135.10672 39.06369 0.0583 5.9 9.9 0.43 L 40.48 0 -0.07 +9.32 J090025.37+390353.7 135.10572 39.06492 0.0582 5.9 10.1 7.22 8.3 40.48 1 -4.22 +74.29 J151806.13+424445.0 229.52558 42.74585 0.0403 6.2 10.8 50.00 8.4 40.13 1 -31.82 J151806.37+424438.1 229.52664 42.74387 0.0407 6.2 LL L 40.14 1 +4.67 J104518.04+351913.1 161.32520 35.32032 0.0676 6.2 10.6 28.86 7.7 40.30 1 -6.07 +38.08 J104518.43+351913.5 161.32682 35.32041 0.0674 6.2 10.6 27.08 7.0 40.30 1 -16.33 +0.46 J090332.77+011236.3 135.88657 1.21009 0.0580 6.3 10.2 6.7 40.33 0 0.17 -0.15 +0.23 J090332.99+011231.7 135.88747 1.20881 0.0579 6.3 9.7 0.09 L 40.33 0 -0.07 +7.89 J133817.27+481632.3 204.57196 48.27564 0.0278 6.4 10.0 5.48 7.8 40.12 1 -3.33 +3.66 J133817.77+481641.1 204.57404 48.27808 0.0277 6.4 10.6 2.84 8.1 40.13 1 -1.66 +14.39 J114753.63+094552.0 176.97346 9.76444 0.0951 6.6 10.3 8.63 8.6 40.93 1 -5.58 +1.56 J114753.68+094555.6 176.97367 9.76544 0.0966 6.6 11.0 1.10 7.7 40.95 0 -0.63 +0.03 J093634.03+232627.0 144.14185 23.44083 0.0284 6.8 10.8 0.00 7.8 40.51 1 -0.00 +0.39 J093633.93+232638.7 144.14144 23.44411 0.0283 6.8 10.5 1.83 6.9 40.50 0 -0.42 +0.20 J123257.15+091756.1 188.23816 9.29892 0.1048 7.3 11.3 0.03 8.2 41.76 0 -0.03 J123257.38+091757.7 188.23912 9.29939 0.1049 7.3 LL L 41.76 0 +0.34 J135853.78+280346.7 209.72413 28.06300 0.0866 7.4 10.1 0.12 L 41.48 0 -0.11 J135853.66+280342.5 209.72362 28.06182 0.0868 7.4 LL L 41.56 0 J084113.09+322459.6 130.30455 32.41657 0.0684 7.7 LL L 39.92 1 +0.17 J084112.79+322455.1 130.30329 32.41533 0.0696 7.7 10.3 0.13 8.0 39.94 0 -0.07 +2.02 J140737.16+442856.2 211.90487 44.48229 0.1429 7.7 10.8 0.80 7.0 41.14 0 -0.62 J140737.43+442855.1 211.90600 44.48200 0.1430 7.7 LL L 41.14 1 +7.55 J084135.08+010156.1 130.39619 1.03228 0.1106 7.8 10.5 5.88 L 40.74 1 -3.44 J084134.87+010153.9 130.39532 1.03165 0.1105 7.8 LL L 40.74 0 +0.41 J230010.24−000533.9 345.04269 −0.09276 0.1798 7.9 11.5 0.06 8.9 41.33 0 -0.06 +0.69 J230010.17−000531.5 345.04239 −0.09211 0.1797 7.9 11.8 0.09 8.8 41.33 1 -0.09 J112536.15+542257.2 171.40067 54.38264 0.0207 7.9 LL L 39.94 1 +0.03 J112535.23+542314.4 171.39682 54.38741 0.0206 7.9 7.1 0.02 L 39.87 0 -0.01 +0.95 J121247.04+070821.6 183.19604 7.13933 0.1362 8.3 L 0.72 L 42.44 0 -0.39 J121246.84+070823.0 183.19517 7.13975 0.1367 8.3 LL L 42.44 0 +0.09 J102229.47+383538.4 155.62280 38.59401 0.0519 8.6 10.9 0.02 7.6 40.35 0 -0.01 +0.02 J102229.95+383544.7 155.62480 38.59577 0.0523 8.6 9.6 0.02 L 40.35 0 -0.01 +0.18 J004343.80+010216.9 10.93251 1.03805 0.1069 8.8 10.4 0.03 7.7 41.98 0 -0.03 +0.87 J004344.07+010215.1 10.93365 1.03754 0.1070 8.8 10.5 2.95 6.9 41.96 0 -0.43 +5.73 J083817.59+305453.5 129.57329 30.91486 0.0478 8.8 10.7 7.0 40.88 1 2.62 -1.88 +0.28 J083817.95+305501.1 129.57479 30.91697 0.0481 8.8 11.2 0.05 7.3 40.89 0 -0.04 4 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 1 (Continued) Name R.A. Decl. zr log M SFR log M Flag p BH log L * X,lim (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) +0.20 J110713.23+650606.6 166.80511 65.10192 0.0328 8.8 11.2 0.03 8.1 40.62 1 -0.03 J110713.49+650553.2 166.80622 65.09819 0.0319 8.8 LL L 40.53 1 +1.61 J090714.45+520343.4 136.81021 52.06206 0.0596 8.9 10.6 0.59 7.7 40.54 1 -0.52 +2.95 J090714.61+520350.7 136.81087 52.06408 0.0602 8.9 10.3 1.28 7.0 40.55 1 -0.94 +0.18 J145309.42+215404.4 223.28929 21.90123 0.1169 9.2 11.0 0.03 8.3 42.18 0 -0.03 +0.09 J145309.62+215407.8 223.29010 21.90220 0.1155 9.2 10.8 0.01 7.9 42.13 0 -0.01 +0.18 J111828.42−003302.7 169.61845 −0.55077 0.1001 9.3 10.8 0.03 7.4 41.62 0 -0.03 +0.22 J111828.41−003307.8 169.61842 −0.55216 0.1003 9.3 10.3 0.60 6.5 41.62 0 -0.16 +0.41 J134736.41+173404.7 206.90171 17.56797 0.0447 9.3 9.4 L 41.17 1 0.30 -0.18 +0.35 J134737.11+173404.1 206.90462 17.56781 0.0450 9.3 10.5 0.13 6.7 41.18 0 -0.11 J142445.68+333749.4 216.19036 33.63039 0.0710 9.5 LL L 41.48 0 +0.59 J142445.86+333742.7 216.19111 33.62855 0.0718 9.5 10.5 2.47 L 41.50 0 -0.41 +0.97 J000249.07+004504.8 0.70446 0.75133 0.0868 9.5 11.2 0.16 8.6 41.60 1 -0.15 +0.09 J000249.44+004506.7 0.70600 0.75186 0.0865 9.5 10.9 0.01 7.8 41.60 0 -0.01 J094543.54+094901.5 146.43146 9.81709 0.1564 9.6 LL L 41.74 1 +36.44 J094543.78+094901.2 146.43245 9.81700 0.1566 9.6 11.2 25.98 7.5 41.64 0 -14.72 +0.11 J143106.40+253800.0 217.77668 25.63335 0.0964 9.7 10.9 0.02 8.1 41.69 0 -0.02 J143106.79+253801.3 217.77832 25.63370 0.0961 9.7 LL L 41.69 0 +2.10 J085953.33+131055.3 134.97224 13.18205 0.0308 9.7 10.6 0.44 6.9 39.98 1 -0.42 +0.01 J085952.51+131044.3 134.96882 13.17900 0.0297 9.7 10.1 0.01 5.8 39.94 0 -0.01 +0.08 J123515.49+122909.0 188.81454 12.48585 0.0485 9.9 10.3 0.01 6.1 40.06 1 -0.01 +0.03 J123516.05+122915.4 188.81688 12.48763 0.0488 9.9 9.5 0.15 L 40.06 0 -0.03 J161758.52+345439.9 244.49387 34.91109 0.1497 10.0 LL L 41.97 1 J161758.62+345436.3 244.49426 34.91007 0.1492 10.0 LL L 41.96 0 +0.31 J125253.91−031811.0 193.22466 −3.30309 0.0863 10.3 10.6 7.1 41.88 0 0.07 -0.06 J125254.33−031812.1 193.22640 −3.30338 0.0862 10.3 LL L 41.88 0 +0.17 J121514.42+130604.5 183.81009 13.10126 0.1227 10.3 11.1 0.03 8.4 41.71 0 -0.03 +0.33 J121514.17+130601.5 183.80906 13.10043 0.1242 10.3 10.9 0.08 7.4 41.76 0 -0.07 +0.26 J095749.15+050638.3 149.45481 5.11066 0.1217 10.7 11.1 0.04 7.9 41.49 1 -0.04 J095748.95+050642.2 149.45399 5.11174 0.1221 10.7 LL L 41.50 0 +0.04 J123637.31+163351.8 189.15549 16.56441 0.0728 10.7 10.8 0.01 6.9 40.62 0 -0.01 +0.11 J123637.50+163344.6 189.15627 16.56239 0.0733 10.7 11.1 0.02 8.5 40.66 1 -0.02 +0.34 J114608.29−010709.8 176.53458 −1.11940 0.1189 11.2 11.1 0.05 8.2 41.28 0 -0.05 J114608.19−010714.8 176.53414 −1.12078 0.1190 11.2 LL L 41.28 0 J151110.35+054851.7 227.79314 5.81437 0.0799 11.8 LL L 40.43 0 +0.04 J151109.85+054849.3 227.79105 5.81370 0.0803 11.8 10.3 0.01 6.6 40.45 0 -0.01 +21.73 J124545.20+010447.5 191.43836 1.07987 0.1068 11.9 11.3 12.39 8.1 41.35 1 -8.11 +0.16 J124545.13+010453.4 191.43807 1.08153 0.1064 11.9 10.9 0.03 7.1 41.34 0 -0.02 J094130.00+412302.0 145.37504 41.38390 0.0174 12.1 LL L 39.76 0 +0.01 J094132.00+412235.5 145.38339 41.37656 0.0172 12.1 8.6 0.01 L 39.73 0 -0.00 +0.09 J090134.48+180942.9 135.39368 18.16195 0.0665 12.2 10.8 7.3 41.39 1 0.01 -0.01 +0.05 J090135.15+180941.7 135.39646 18.16159 0.0665 12.2 9.7 0.07 L 41.63 0 -0.02 J105622.07+421807.8 164.09208 42.30219 0.0775 12.3 LL L 39.90 1 +0.08 J105622.82+421809.7 164.09518 42.30267 0.0776 12.3 10.3 0.02 L 39.90 0 -0.02 J132924.60+114816.5 202.35253 11.80459 0.0222 12.4 LL L 39.28 0 +0.57 J132924.25+114749.3 202.35108 11.79703 0.0216 12.4 10.5 1.31 6.4 39.25 1 -0.41 J111136.07+574952.4 167.90019 57.83131 0.0472 12.5 LL L 41.11 0 +0.19 J111134.88+574942.8 167.89524 57.82866 0.0465 12.5 9.9 0.46 L 41.27 0 -0.13 +0.12 J135429.06+132757.3 208.62108 13.46592 0.0633 12.5 10.1 0.04 7.0 40.82 1 -0.04 +1.66 J135429.18+132807.4 208.62158 13.46872 0.0634 12.5 10.7 0.64 7.0 40.82 0 -0.52 +0.02 J125725.84+273246.0 194.35769 27.54613 0.0186 12.5 10.2 0.00 7.6 39.23 1 -0.00 +0.00 J125723.56+273259.7 194.34822 27.54993 0.0201 12.5 9.0 0.00 L 39.32 0 -0.00 J011448.67−002946.0 18.70281 −0.49612 0.0338 12.6 LL L 40.18 1 +0.03 J011449.81−002943.6 18.70760 −0.49542 0.0349 12.6 L 0.16 L 40.15 0 -0.03 +4.45 J145051.50+050652.1 222.71458 5.11448 0.0275 12.6 11.0 3.24 7.5 39.92 1 -1.94 J145050.63+050710.8 222.71097 5.11968 0.0282 12.6 LL L 39.94 1 J075311.87+123749.1 118.29946 12.63031 0.0298 12.9 LL L 39.87 0 +0.01 J075313.34+123749.1 118.30561 12.63031 0.0294 12.9 8.3 0.23 L 39.86 0 -0.04 5 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 1 (Continued) Name R.A. Decl. zr log M SFR log M Flag p BH log L * X,lim (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) J134844.49+271044.7 207.18540 27.17911 0.0599 12.9 LL L 40.23 1 +0.08 J134844.48+271055.9 207.18535 27.18220 0.0596 12.9 9.9 0.02 L 40.23 0 -0.02 J141958.98+060320.1 214.99577 6.05560 0.0473 13.4 LL L 40.64 0 +0.02 J141959.06+060305.7 214.99610 6.05161 0.0472 13.4 9.5 0.02 L 40.63 0 -0.01 +0.20 J090005.15+391952.2 135.02159 39.33120 0.0959 14.0 11.3 0.03 8.2 41.62 1 -0.03 J090005.69+391947.4 135.02384 39.32988 0.0968 14.0 LL L 41.64 0 +2.23 J125315.57−031030.2 193.31490 −3.17507 0.0845 14.1 10.4 1.52 6.1 41.71 0 -0.93 +0.43 J125315.99−031036.4 193.31665 −3.17680 0.0852 14.1 10.3 0.74 L 41.66 1 -0.26 +0.07 J135225.64+142919.3 208.10683 14.48869 0.0415 14.1 10.7 0.01 7.4 40.83 0 -0.01 +7.98 J135226.65+142927.5 208.11104 14.49097 0.0406 14.1 11.2 3.74 7.8 40.81 0 -2.65 J141807.91+073232.5 214.53297 7.54237 0.0239 14.2 LL L 40.28 0 +0.02 J141805.96+073226.7 214.52486 7.54077 0.0234 14.2 9.3 0.00 L 40.27 0 -0.00 J125400.79+462752.4 193.50335 46.46458 0.0610 14.5 LL L 41.43 0 +2.06 J125359.62+462750.2 193.49846 46.46397 0.0614 14.5 10.3 5.8 41.37 1 0.68 -0.63 +0.07 J163026.65+243640.2 247.61105 24.61118 0.0623 14.7 10.8 0.01 7.4 40.87 0 -0.01 +0.04 J163026.85+243652.1 247.61189 24.61449 0.0619 14.7 10.0 0.01 L 40.87 0 -0.01 +0.03 J080133.07+141341.6 120.39136 14.22618 0.0538 14.8 9.3 0.22 L 40.00 0 -0.03 +0.08 J080133.94+141334.0 120.38784 14.22821 0.0529 14.8 9.7 0.46 L 39.99 1 -0.11 +0.11 J144804.16+182537.8 222.01737 18.42718 0.0378 15.1 10.6 0.02 7.1 40.00 1 -0.02 +0.01 J144804.23+182558.0 222.01764 18.43277 0.0390 15.1 9.6 0.01 L 40.02 0 -0.00 J151031.75+060007.0 227.63229 6.00195 0.0800 15.2 LL L 41.67 0 +0.05 J151031.66+055957.0 227.63192 5.99919 0.0801 15.2 10.4 0.01 7.4 41.66 0 -0.01 +0.22 J141115.91+573609.0 212.81623 57.60258 0.1062 15.2 11.5 0.03 8.6 41.37 1 -0.03 +0.11 J141115.95+573601.2 212.81638 57.60041 0.1049 15.2 10.7 0.02 8.4 41.36 0 -0.02 J111627.21+570659.1 169.11338 57.11651 0.0469 15.2 LL L 40.98 0 +0.01 J111625.68+570709.8 169.10697 57.11950 0.0464 15.2 9.3 0.24 L 41.00 0 -0.04 +0.50 J115532.11+583532.5 178.88379 58.59246 0.1644 15.4 11.1 0.10 8.1 42.72 0 -0.09 +0.37 J115532.10+583538.0 178.88375 58.59397 0.1634 15.4 11.1 0.06 8.2 42.55 0 -0.06 J142553.53+340452.6 216.47307 34.08129 0.0726 15.4 LL L 40.69 0 +0.07 J142553.20+340442.2 216.47172 34.07840 0.0733 15.4 10.5 0.01 7.1 40.76 0 -0.01 J125917.25−013427.8 194.82191 −1.57440 0.1682 15.5 LL L 42.39 0 +19.47 J125917.14−013422.6 194.82143 −1.57297 0.1679 15.5 10.9 14.68 7.0 42.50 0 -8.08 +0.00 J120429.88+022654.6 181.12451 2.44849 0.0200 15.6 9.1 0.00 L 40.09 0 -0.00 +0.00 J120432.18+022711.1 181.13413 2.45310 0.0200 15.6 9.6 0.00 L 39.98 0 -0.00 +0.03 J125922.72+312213.7 194.84467 31.37050 0.0526 15.8 9.7 0.04 L 40.63 0 -0.02 +0.04 J125922.03+312201.1 194.84180 31.36698 0.0524 15.8 9.9 0.04 L 40.63 0 -0.02 J133525.37+380533.9 203.85570 38.09276 0.0655 16.1 LL L 40.97 1 +0.04 J133525.26+380538.6 203.85305 38.09515 0.0649 16.1 10.0 0.01 5.7 40.96 0 -0.01 +0.49 J142442.81−015929.8 216.17840 −1.99163 0.1746 16.8 11.8 0.07 8.3 42.72 0 -0.07 +8.22 J142442.91−015924.3 216.17881 −1.99011 0.1742 16.8 11.2 7.9 42.88 0 5.82 -3.31 J143541.79+330820.0 218.92417 33.13891 0.1206 16.9 LL L 42.01 1 +0.16 J143542.38+330822.1 218.92666 33.13947 0.1205 16.9 11.2 0.03 7.3 42.01 0 -0.02 +0.03 J085405.94+011111.4 133.52477 1.18650 0.0447 17.0 9.4 0.32 L 40.38 0 -0.06 +0.06 J085405.90+011130.6 133.52459 1.19186 0.0441 17.0 10.2 0.43 6.0 40.37 0 -0.06 +0.17 J102108.45+482855.4 155.28523 48.48206 0.0618 17.1 10.5 0.13 7.8 40.78 0 -0.07 +0.47 J102109.88+482857.2 155.29119 48.48256 0.0615 17.1 10.1 0.18 L 40.78 1 -0.14 +0.04 J111519.23+542310.9 168.83012 54.38636 0.0713 17.1 10.5 0.01 7.6 40.57 0 -0.01 +5.20 J111519.98+542316.7 168.83325 54.38797 0.0704 17.1 11.1 1.72 8.0 40.62 1 -1.60 J112402.95+430901.0 171.01229 43.15028 0.0715 17.3 LL L 40.36 1 +0.63 J112401.84+430857.2 171.00768 43.14922 0.0709 17.3 10.5 1.07 6.9 40.33 1 -0.38 +0.08 J171255.40+640145.3 258.23079 64.02934 0.0811 17.5 10.5 0.01 6.7 40.63 0 -0.01 +0.10 J171255.44+640156.7 258.23090 64.03252 0.0813 17.5 10.1 0.10 L 40.60 0 -0.04 J090215.15+520754.7 135.56311 52.13189 0.1029 17.7 LL L 40.95 0 +0.31 J090215.79+520802.0 135.56578 52.13393 0.1023 17.7 10.2 1.06 L 40.99 1 -0.14 J110418.11+594831.6 166.07545 59.80882 0.1148 17.8 LL L 41.38 0 +0.53 J110419.26+594830.7 166.08019 59.80861 0.1132 17.8 10.3 0.43 5.6 41.42 0 -0.21 +0.07 J143454.22+334934.5 218.72592 33.82625 0.0578 18.0 10.8 0.01 7.3 41.06 0 -0.01 +1.82 J143454.68+334920.0 218.72783 33.82222 0.0587 18.0 10.7 0.64 6.7 41.06 0 -0.58 6 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 1 (Continued) Name R.A. Decl. zr log M SFR log M Flag p BH log L * X,lim (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) J155207.85+273514.6 238.03275 27.58740 0.0747 18.4 LL L 40.20 1 +0.13 J155207.87+273501.6 238.03282 27.58380 0.0748 18.4 11.2 0.02 8.4 40.28 1 -0.02 +0.13 J083902.97+470756.3 129.76239 47.13233 0.0524 18.6 10.7 0.03 7.3 40.62 1 -0.02 +0.30 J083902.50+470814.0 129.76046 47.13722 0.0534 18.6 10.5 1.21 7.6 40.64 0 -0.17 +10.33 J214622.41+000452.1 326.59337 0.08114 0.0754 18.7 10.4 4.72 6.1 41.22 0 -3.39 +6.49 J214623.23+000456.7 326.59679 0.08242 0.0750 18.7 10.9 2.72 7.2 41.22 1 -2.04 J123042.83+103445.3 187.67848 10.57926 0.1636 19.6 LL L 41.88 0 +0.38 J123043.27+103442.9 187.68033 10.57860 0.1636 19.6 11.5 0.06 8.7 41.86 0 -0.06 J161111.72+522645.6 242.79888 52.44607 0.0605 19.7 LL L 40.90 0 +0.81 J161113.52+522649.3 242.80639 52.44709 0.0607 19.7 10.3 0.92 L 40.86 1 -0.41 Note. (1) SDSS names with J2000 coordinates given in the form of “hhmmss.ss+ddmmss.s;” (2)–(3) optical position of the galaxy nucleus; (4) redshift; (5) projected −1 physical separation of galaxies in each pair, in units of kpc; (6) stellar mass, in units of M ; (7) star formation rate, in units of M yr , given by the MPA-JHU DR7 e e catalog; (8) black hole mass estimate inferred from σ assuming the M –σ relation of Gültekin et al. (2009), in units of M ; (9) 0.5–8 keV limiting luminosity for BH e * * −1 source detection, in units of erg s ; (10) flag for X-ray detection, 1 and 0 represent detection and non-detection in X-ray, respectively. (This table is available in machine-readable form.) −6 PSF maps supplied and a false detection probability of 10 . of a given band at the position of each nucleus, following the We then searched for an X-ray counterpart of each optical method of Kashyap et al. (2010). Figure 2 plots the histogram nucleus from the X-ray source lists output by wavdetect, of the 0.5–8 limiting luminosity for both the current sample adopting a matching radius of 2″, an empirically optimal value (listed in Table 1) and the AGN pairs of Hou et al. (2020), given the angular resolution and astrometry accuracy of which have a similar distribution, facilitating a direct comparison between the two sample. Chandra in most cases. This is further justified by a random matching test by artificially shifting the positions of all nuclei by ±10″ in R.A. and decl., which finds on average less than 3.3. NuSTAR Spectral Analysis one coincident match with the detected X-ray sources. We note that no pair in our sample has angular separation less than this To help constrain the presence of intrinsically luminous but matching radius (see the third panel in Figure 1), which means heavily obscured AGNs in the sample galaxies, we utilized two nuclei in a pair would not be matched with one identical archival NuSTAR observations that are sensitive to the hard X-ray counterpart in any case. If the optical nucleus was (10 keV) X-rays from obscured AGNs. Eight pairs in the matched with an X-ray counterpart in any of the three energy current sample have been observed by NuSTAR, with an bands, we consider it to be X-ray detected. effective exposure ranging from 19.5 to 211.3 ks. We note that Source photometry was then calculated using the CIAO tool half of these eight observations were taken as a targeted aprate, which properly handles the counting statistics in the observation to probe the hard X-ray emission from a low-count regime. Source count at a given band was extracted putative AGN. from within the 90% enclosed count radius (ECR). The local The NuSTAR data were downloaded and reprocessed background was evaluated from a concentric annulus with following the standard nupipeline in the software package inner-to-outer radii 2–5 times the 90% ECR for the inner NuSTARDAS v2.1.2. The spectra of each galaxy pair were radius, excluding pixels falling within the 90% ECR of extracted for both focal plane modules A and B (FPMA and neighboring sources, if any. In a few cases where the two FPMB) with nuproducts. A circular region was used to extract nuclei have overlapping 90% ECR, we adopt the 50% ECR for the source spectrum, which has a radius of 60″, approximately photometry. The net photon flux was derived by dividing the equaling 75% ECR. Since the two nuclei in a given pair are not exposure map and corrected for the ECF. well resolved by NuSTAR, the source center was set to be the For the optical nuclei without an X-ray counterpart found by brighter nucleus as seen by Chandra, which is generally wavdetect, we extracted the source and background counts in a consistent with the peak of the NuSTAR-detected signal. The similar way and estimated a 3σ upper limit of the net photon background spectra were extracted from a concentric annulus flux using aprate. If the 3σ lower limit were greater than zero, with an inner radius of 90″ and an outer radius of 150″. It turns the nucleus is regarded as an X-ray detection. Using this more out that three of the eight pairs show no significant signal above quantitative criterion, we recover a few more nuclei with the background, thus they were neglected in the spectral significant X-ray emission that have been filtered by wavdetect. analysis. For the remaining nuclei, we again used aprate to derive a 3σ For the five pairs with significant hard X-ray emission, the upper limit of the net photon flux. spectra were grouped to achieve a signal-to-noise ratio (S/N) The net photon fluxes (or upper limits) were then converted greater than 3 per bin over the energy range of 3–79 keV. We to an unabsorbed luminosity in the 0.5–2 and 2–10 keV bands, follow the method of Zappacosta et al. (2018) to simulate by multiplying a unique conversion factor for a given energy background spectrum using the software NUSKYBGD (Wik band according to the fiducial incident spectrum described in et al. 2014) to account for the spatially dependent background Section 3.1. The net counts, photon fluxes, and luminosities are of NuSTAR. This task aims to compute the relative strengths of listed in Table 2. We have also determined the detection limit different background components and hence well reproduce the 7 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 2 X-Ray Properties of Close Galaxy Pairs Name XR.A. XDecl. Counts F F log L log L HR 0.5−2 2-8 0.5−2 2−10 (1)(2)(3)(4)(5)(6)(7)(8)(9) +8.3 +0.42 +0.26 +0.07 +0.13 +0.02 J102700.56+174900.3 156.75230 17.81692 55.1 2.30 0.79 40.73 40.85 -0.76 -8.2 -0.38 -0.22 -0.08 -0.14 -0.24 +4.9 +0.67 +0.10 +0.00 J085837.53+182221.6 134.65689 18.37266 18.7 2.55 <0.19 40.66 <40.13 -0.95 -4.3 -0.58 -0.11 -0.05 +4.4 +0.57 +0.25 +0.12 +0.42 +0.04 J085837.68+182223.4 134.65689 18.37266 14.2 1.79 0.15 40.51 40.04 -0.83 -3.7 -0.48 -0.13 -0.14 -0.75 -0.17 +2.3 +0.35 +0.34 +0.26 J105842.44+314457.6 164.67683 31.74933 2.6 0.30 <0.46 39.92 <40.70 -0.26 -1.6 -0.23 -0.65 -0.74 +0.16 +10.4 +0.60 +2.00 +0.04 +0.08 J105842.58+314459.8 164.67744 31.74995 96.1 1.36 18.40 40.57 42.30 0.82 -10.3 -0.48 -2.10 -0.19 -0.05 -0.05 +6.0 +0.29 +0.18 +0.01 J002208.69+002200.5 5.53621 0.36681 11.9 0.58 <0.13 40.19 <40.12 -0.87 -4.8 -0.23 -0.22 -0.13 +5.2 +0.22 +0.19 +0.26 +0.37 +0.17 J002208.83+002202.8 5.53679 0.36744 7.8 0.28 0.14 39.87 40.17 -0.41 -4.0 -0.16 -0.13 -0.37 -0.88 -0.59 +4.8 +0.75 +0.57 +0.12 +0.18 +0.16 J133032.00−003613.5 202.63333 −0.60379 17.3 2.31 1.12 40.54 40.82 -0.42 -4.1 -0.63 -0.45 -0.14 -0.23 -0.25 +36.4 +1.50 +1.50 +0.02 +0.02 +0.03 J141447.15−000013.3 213.69646 −0.00376 1188.2 27.60 40.40 41.50 42.26 0.04 -36.0 -1.50 -1.50 -0.02 -0.02 -0.04 +25.0 +1.40 +0.80 +0.03 +0.03 +0.04 J141447.48−000011.3 213.69783 −0.00321 556.3 21.70 11.90 41.40 41.73 -0.42 -24.7 -1.30 -0.90 -0.03 -0.03 -0.04 +24.2 +3.40 +9.16 +0.06 +0.02 +0.03 J235654.30−101605.4 359.22646 −10.26818 526.7 24.30 188.03 41.85 43.33 0.74 -24.0 -3.40 -9.06 -0.07 -0.02 -0.03 +6.3 +21.20 +36.53 +0.34 +0.17 +0.43 J122814.15+442711.7 187.05896 44.45325 14.3 17.80 73.90 40.68 41.89 0.57 -5.2 -13.85 -28.40 -0.65 -0.21 -0.11 +6.0 +34.93 +0.17 +0.18 J122815.23+442711.3 187.06348 44.45314 12.1 71.60 <25.80 41.27 <41.42 -0.42 -4.8 -27.40 -0.21 -0.58 +76.6 +9.51 +10.47 +0.01 +0.01 +0.01 J112648.50+351503.2 171.70213 35.25081 5273.1 469.87 500.68 42.39 43.01 -0.13 -75.9 -9.41 -10.37 -0.01 -0.01 -0.01 +3.9 +0.64 +0.15 +0.03 J112648.65+351454.2 171.70263 35.24838 9.5 1.54 <0.43 39.90 <39.94 -0.83 -3.2 -0.53 -0.18 -0.17 +2.9 +0.30 +0.59 +0.26 +0.18 +0.41 J090025.37+390353.7 135.10567 39.06513 7.8 0.35 1.16 39.79 40.90 0.37 -2.8 -0.21 -0.47 -0.40 -0.22 -0.21 +9.4 +1.30 +0.83 +0.05 +0.13 +0.07 J151806.13+424445.0 229.52559 42.74580 76.4 10.20 2.44 40.92 40.90 -0.68 -9.3 -1.35 -0.69 -0.06 -0.14 -0.10 +4.5 +0.70 +0.11 +0.00 J151806.37+424438.1 229.52648 42.74393 15.1 2.36 <0.26 40.30 <39.93 -0.93 -3.9 -0.61 -0.13 -0.07 +6.1 +0.48 +0.25 +0.10 +0.16 +0.11 J104518.04+351913.1 161.32538 35.32022 28.2 1.84 0.58 40.64 40.74 -0.60 -5.5 -0.42 -0.20 -0.11 -0.19 -0.16 +4.2 +0.31 +0.24 +0.19 +0.16 +0.25 J104518.43+351913.5 161.32676 35.32023 10.6 0.55 0.52 40.11 40.69 -0.11 -3.5 -0.24 -0.19 -0.26 -0.20 -0.38 +10.6 +2.10 +1.47 +0.05 +0.11 +0.07 J133817.27+481632.3 204.57207 48.27566 92.6 16.30 4.94 40.80 40.87 -0.61 -10.5 -2.00 -1.25 -0.06 -0.13 -0.11 +15.5 +2.60 +3.10 +0.04 +0.05 +0.06 J133817.77+481641.1 204.57415 48.27808 209.4 26.00 27.40 41.00 41.61 -0.17 -15.3 -2.50 -3.10 -0.04 -0.05 -0.08 +60.7 +4.55 +7.45 +0.02 +0.01 +0.02 J114753.63+094552.0 176.97337 9.76444 3302.0 112.86 361.19 42.74 43.84 0.44 -60.1 -4.51 -7.38 -0.02 -0.01 -0.02 +3.3 +2.39 +1.26 +0.16 +0.41 +0.05 J093634.03+232627.0 144.14171 23.44080 7.6 5.22 0.79 40.32 40.10 -0.75 -2.7 -1.93 -0.67 -0.20 -0.81 -0.25 +14.6 +0.36 +0.28 +0.04 +0.06 +0.06 J084113.09+322459.6 130.30458 32.41649 187.9 4.00 1.84 40.99 41.25 -0.48 -14.5 -0.35 -0.27 -0.04 -0.07 -0.07 +34.5 +2.40 +3.00 +0.03 +0.02 +0.03 J140737.43+442855.1 211.90597 44.48199 1064.1 38.30 79.30 42.65 43.56 0.21 -34.1 -2.30 -3.00 -0.03 -0.02 -0.03 +20.3 +1.00 +1.10 +0.03 +0.03 +0.06 J084135.08+010156.1 130.39612 1.03229 366.7 12.60 14.30 41.93 42.58 -0.08 -20.1 -1.00 -1.00 -0.04 -0.03 -0.05 +4.7 +0.43 +0.18 +0.14 +0.28 +0.08 J230010.17−000531.5 345.04272 −0.09205 14.7 1.10 0.19 41.32 41.16 -0.73 -4.0 -0.35 -0.12 -0.17 -0.40 -0.21 +65.3 +17.37 +17.06 +0.01 +0.01 +0.02 J112536.15+542257.2 171.40069 54.38269 3830.9 785.97 635.29 42.22 42.72 -0.19 -64.7 -17.19 -16.89 -0.01 -0.01 -0.02 +2.9 +2.17 +1.05 +0.15 +0.37 +0.06 J083817.59+305453.5 129.57323 30.91485 0.77 40.79 -0.73 7.8 5.26 40.55 -2.8 -2.04 -0.60 -0.21 -0.66 -0.26 +2.6 +1.59 +1.38 +0.29 +0.28 +0.33 J110713.23+650606.6 166.80544 65.10198 4.3 1.69 1.50 39.96 40.50 -0.15 -2.0 -1.05 -0.90 -0.42 -0.40 -0.58 +2.3 +1.84 +1.05 +0.23 +0.37 +0.11 J110713.49+650553.2 166.80633 65.09846 3.3 2.61 0.77 40.13 40.19 -0.53 -1.7 -1.32 -0.60 -0.31 -0.66 -0.47 +7.1 +0.54 +1.39 +0.19 +0.08 +0.15 J090714.45+520343.4 136.81026 52.06206 40.7 1.01 7.36 40.27 41.73 0.69 -6.4 -0.40 -1.24 -0.22 -0.08 -0.09 +11.6 +1.05 +2.10 +0.08 +0.04 +0.09 J090714.61+520350.7 136.81087 52.06413 120.9 4.93 19.60 40.97 42.16 0.52 -11.5 -0.97 -2.10 -0.10 -0.05 -0.07 +8.7 +11.50 +4.56 +0.05 +0.17 +0.04 J134736.41+173404.7 206.90178 17.56801 67.8 86.40 9.84 41.95 41.59 -0.82 -8.6 -11.50 -4.12 -0.06 -0.24 -0.09 +5.2 +2.76 +3.09 +0.18 +0.15 +0.28 J000249.07+004504.8 0.70433 0.75128 18.1 5.54 7.21 41.35 42.06 -0.08 -6.3 -2.61 -3.11 -0.28 -0.25 -0.34 +6.4 +0.78 +0.71 +0.31 +0.19 +0.49 J094543.54+094901.5 146.43146 9.81709 13.5 0.74 1.27 41.02 41.84 0.10 -5.2 -0.50 -0.56 -0.49 -0.25 -0.43 +23.1 +0.55 +4.40 +0.21 +0.02 +0.02 J085953.33+131055.3 134.97212 13.18192 477.5 0.88 89.20 39.63 42.22 0.97 -22.8 -0.41 -4.30 -0.27 -0.02 -0.01 +9.7 +0.33 +0.30 +0.15 +0.17 +0.24 J123515.49+122909.0 188.81481 12.48569 31.4 0.82 0.64 40.00 40.48 -0.25 -8.4 -0.28 -0.24 -0.18 -0.21 -0.27 +3.5 +1.43 +0.31 +0.04 J161758.52+345439.9 244.49387 34.91109 3.0 1.39 <0.95 41.25 <41.68 -0.64 -2.3 -0.92 -0.47 -0.36 +5.2 +0.71 +0.68 +0.41 +0.21 +0.79 J095749.15+050638.3 149.45481 5.11066 9.4 0.45 1.08 40.57 41.54 0.21 -0.38 -0.84 -0.27 -0.26 -4.0 -0.50 +13.8 +1.30 +0.37 +0.04 +0.14 +0.02 J123637.50+163344.6 189.15634 16.56247 163.2 15.00 0.96 41.63 41.03 -0.90 -13.7 -1.30 -0.30 -0.04 -0.16 -0.04 +3.1 +0.98 +0.16 +0.01 J124545.20+010447.5 191.43838 1.08009 6.6 2.13 <0.59 41.12 <41.16 -0.85 -2.5 -0.79 -0.20 -0.15 +5.9 +2.90 +0.09 +0.08 J090134.48+180942.9 135.39369 18.16188 27.3 <1.07 12.60 <40.39 42.06 0.92 -5.2 -2.50 -0.10 -0.01 +7.4 +0.11 +0.05 +0.08 +0.15 +0.10 J105622.07+421807.8 164.09197 42.30219 41.1 0.56 0.14 40.25 40.23 -0.68 -6.7 -0.11 -0.05 -0.09 -0.18 -0.12 +4.8 +0.28 +0.17 +0.12 +0.29 +0.10 J132924.25+114749.3 202.35106 11.79699 16.7 0.90 0.18 39.32 39.22 -0.70 -4.2 -0.24 -0.11 -0.13 -0.42 -0.23 +16.2 +1.13 +5.30 +0.15 +0.03 +0.04 J135429.06+132757.3 208.62108 13.46604 234.2 2.80 75.10 40.77 42.79 0.91 -16.0 -0.92 -5.20 -0.17 -0.03 -0.02 +7.1 +0.25 +0.16 +0.17 +0.17 +0.26 J125725.84+273246.0 194.35769 27.54613 21.4 0.52 0.33 38.95 39.35 -0.33 -6.4 -0.22 -0.14 -0.23 -0.24 -0.34 +35.0 +6.64 +4.50 +0.02 +0.02 +0.03 J011448.67−002946.0 18.70286 −0.49634 1097.0 163.66 86.20 41.98 42.29 -0.40 -34.6 -6.57 -4.40 -0.02 -0.02 -0.03 +15.3 +3.10 +1.33 +0.03 +0.06 +0.05 J145051.50+050652.1 222.71453 5.11454 208.4 38.20 9.17 41.16 41.13 -0.68 -15.1 -3.10 -1.53 -0.04 -0.08 -0.05 +1.08 +6.4 +0.96 +0.11 +0.11 +0.18 J145050.63+050710.8 222.71082 5.11957 32.9 3.81 3.41 40.18 40.73 -0.17 -5.8 -0.93 -0.83 -0.12 -0.12 -0.17 +4.6 +0.17 +0.12 +0.19 +0.23 +0.30 J134844.49+271044.7 207.18541 27.17911 10.9 0.31 0.18 39.77 40.12 -0.40 -3.9 -0.13 -0.10 -0.24 -0.34 -0.38 +3.5 +2.00 +0.15 +0.01 J090005.15+391952.2 135.02133 39.33119 8.5 4.97 <0.88 41.39 <41.23 -0.89 -2.9 -1.64 -0.17 -0.11 +2.0 +2.51 +0.26 +-0.01 J125315.99−031036.4 193.31665 −3.17680 2.0 3.00 <0.91 41.07 <41.14 -0.84 -1.3 -1.81 -0.40 -0.16 +3.8 +4.90 +2.52 +0.10 +0.38 +0.04 J125359.62+462750.2 193.49847 46.46392 15.6 19.20 1.83 41.58 41.15 -0.81 -4.3 -5.40 -1.43 -0.14 -0.66 -0.19 +3.3 +0.21 +0.14 +0.17 +0.31 +0.10 J080133.94+141334.0 120.38814 14.22832 7.4 0.44 0.14 39.80 39.89 -0.59 -2.7 -0.17 -0.09 -0.21 -0.47 -0.41 +4.4 +0.47 +0.36 +0.15 +0.24 +0.19 J144804.16+182537.8 222.01737 18.42721 14.0 1.16 0.48 39.93 40.13 -0.50 -3.7 -0.38 -0.26 -0.18 -0.33 -0.29 8 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 2 (Continued) Name XR.A. XDecl. Counts F F log L log L HR 0.5−2 2-8 0.5−2 2−10 (1)(2)(3)(4)(5)(6)(7)(8)(9) +5.4 +1.03 +0.19 +0.03 J141115.91+573609.0 212.81623 57.60258 20.2 1.83 <0.76 41.05 <41.27 -0.77 -4.8 -0.80 -0.25 -0.23 +11.4 +2.16 +1.55 +0.09 +0.12 +0.12 J133525.37+380533.9 203.85554 38.09311 62.8 9.54 4.73 41.33 41.62 -0.42 -11.2 -1.90 -1.31 -0.10 -0.14 -0.17 +4.2 +2.98 +2.09 +0.29 +0.36 +0.22 J143541.79+330820.0 218.92417 33.13891 5.8 3.15 1.61 41.41 41.71 -0.36 -3.0 -1.93 -1.27 -0.41 -0.68 -0.59 +2.9 +0.59 +0.51 +0.24 +0.45 +0.14 J102109.88+482857.2 155.29119 48.48256 5.2 0.81 0.28 40.20 40.33 -0.50 -2.2 -0.41 -0.27 -0.31 -1.58 -0.50 +40.9 +1.70 +6.22 +0.04 +0.01 +0.02 J111519.98+542316.7 168.83312 54.38789 1498.0 17.30 218.16 41.65 43.35 0.82 -40.5 -1.60 -6.16 -0.04 -0.01 -0.02 +7.1 +0.27 +0.13 +0.11 +0.21 +0.16 J112402.95+430901.0 171.01226 43.15025 29.2 0.97 0.22 40.42 40.37 -0.72 -6.5 -0.24 -0.12 -0.12 -0.32 -0.18 +5.2 +0.17 +0.11 +0.17 +0.28 +0.11 J112401.84+430857.2 171.00768 43.14922 12.0 0.35 0.12 39.97 40.09 -0.61 -4.6 -0.14 -0.09 -0.22 -0.60 -0.39 +3.1 +0.23 +0.22 +0.24 +0.28 +0.37 J090215.79+520802.0 135.56578 52.13393 5.8 0.32 0.24 40.26 40.73 -0.30 -2.4 -0.16 -0.15 -0.31 -0.45 -0.49 +4.4 +0.15 +0.09 +0.20 +0.29 +0.10 J155207.85+273514.6 238.03275 27.58741 9.1 0.26 0.10 39.88 40.04 -0.48 -3.7 -0.12 -0.06 -0.29 -0.49 -0.52 +16.3 +0.64 +0.40 +0.03 +0.05 +0.05 J155207.87+273501.6 238.03274 27.58389 234.2 7.68 3.14 41.36 41.56 -0.52 -16.1 -0.62 -0.40 -0.04 -0.06 -0.06 +8.9 +0.86 +2.60 +0.19 +0.05 +0.11 J083902.97+470756.3 129.76228 47.13214 70.6 1.58 20.00 40.35 42.04 0.78 -8.8 -0.67 -2.60 -0.24 -0.06 -0.07 +3.3 +0.94 +0.17 +0.22 J214623.23+000456.7 326.59679 0.08242 6.3 <0.49 1.93 <40.16 41.35 0.78 -2.7 -0.76 -0.22 -0.03 +3.9 +0.75 +0.64 +0.17 +0.22 +0.21 J161113.52+522649.3 242.80594 52.44716 10.4 1.54 0.98 40.47 40.87 -0.32 -3.2 -0.59 -0.48 -0.21 -0.29 -0.39 Note. (1) SDSS names with J2000 coordinates given in the form of “hhmmss.ss+ddmmss.s;” (2)–(3) centroid position of the X-ray counterpart; (4) observed net −6 −2 −1 counts in 0.5–8 (F) keV bands; (5)–(6) observed photon flux in 0.5–2 (S) and 2–8 (H) keV bands, in units of 10 ph cm s ; (7)–(8) 0.5–2 and 2–10 keV −1 unabsorbed luminosities, in units of erg s ; (9) hardness ratio between the 0.5–2 and 2–8 keV bands. (This table is available in machine-readable form.) Gaussian component was not required for the other four sources. The spectral fit results are listed in Table 3, which include the best-fit absorption column density, photon index, 3–79 keV unabsorbed flux and 2–10 keV unabsorbed lumin- osity converted from the best-fit model. 4. Results 4.1. X-Ray Detection Rate A bright X-ray source matched with the galactic nucleus usually refers to an AGN, but with potential contamination from the host galaxy (i.e., nuclear starburst). As estimated in Section 4.2, The X-ray emission due to star formation is neglectable compared to AGN, especially for the nuclei with 41 −1 L > 10 erg s . 2−10 In total, we find 70 X-ray-detected nuclei, among which 67 are detected in the S band, 58 are detected in the H band, and 70 are detected in the F band. Among the 92 close galaxy pairs, 14 pairs have both nuclei detected, 42 pairs have only one of the two nuclei detected, and 36 pairs have no X-ray detection. Figure 3 displays the SDSS gri color-composite images and the Chandra 0.5–8 keV images of the newly-found close galaxy Figure 2. 0.5–8 keV detection limit distribution of the close galaxy pairs pairs with both nuclei detected in the X-rays (the other six pairs studied in this work (black solid histogram), in comparison with the close AGN have been studied and presented in Hou et al. 2020). These 16 pairs (blue dashed) in Hou et al. (2020). The vertical lines mark the median value of the individual samples. nuclei have a 0.5–8 keV luminosity ranging from 40 42 −1 1.3 × 10 –6.3 × 10 erg s . +5% We find an X-ray detection rate of 38% (70/184) among background spectrum at each position of the detector. The -5% the 92 close galaxy pairs. The quoted error takes into account FPMA/FPMB spectra were jointly fitted. Spectral analysis was the counting (Poisson) error in both the numerator and carried out with Xspec v.12.12.1c, adopting the χ statistics to denominator. In the more conservative case, where we only determine the best-fit model. Since we are mainly interested in consider X-ray counterparts with a 2–10 keV unabsorbed constraining the line-of-sight absorption column density and 41 −1 luminosity L > 10 erg s , which are most likely domi- the intrinsic X-ray luminosity, we adopted a phenomenological 2−10 nated by an AGN (see Section 4.2), the detection rate becomes model, an absorbed power law model tbabs∗powerlaw, as the +3% 18% (32/184). For comparison, Hou et al. (2020) gave a default model. In one source, J1451+0507, significant excess is -3% +5% present around 6.4 keV, which can be interpreted as an iron detection rate of 27% (36/134) for their entire sample of -5% fluorescent emission line often seen in luminous AGNs. For AGNs (i.e., regardless of the value of r ) above the threshold of 41 −1 this source, we added a Gaussian component to account for the L = 10 erg s . This factor of ∼1.5 difference may be 2−10 putative Fe line, which significantly improved the fit. Such a understood as a systematically higher fraction of true AGNs in 9 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 3 NuSTAR Spectral Fit Results Name Observation ID N Γ χ /d. o. f F L L H 3−79 2−10,N 2−10,C (1)(2)(3)(4)(5)(6)(7)(8) +7.15 +0.25 +1.78 +0.12 +0.03 J0841+0102 60401002002 0.03 0.61 37.48/48 8.82 43.16 42.58 -0.03 -0.13 -2.05 -0.05 -0.03 +0.13 +2.50 +0.73 +0.06 +0.01 J1125+5423 60160430002 0.78 1.59 199.88/203 8.11 42.34 42.72 -0.78 -0.09 -0.67 -0.04 -0.01 +10.08 +0.41 +1.03 +0.19 +0.05 J1338+4816 60465005002 2.09 1.3 42.13/38 3.17 42.02 41.61 -2.09 -0.25 -0.77 -0.10 -0.05 +4.59 +0.14 +1.32 +0.07 +0.03 J1354+1328 60160565002 17.53 1.45 166.38/183 14.81 43.51 42.79 -3.99 -0.13 -1.19 -0.07 -0.03 +17.78 +0.40 +6.10 +0.18 +0.06 J1450+0507 60301025002 0.02 -0.27 32.88/35 13.08 41.50 41.13 -0.02 -0.32 -4.59 -0.07 -0.08 22 −2 2 Note. (1) Source name; (2) NuSTAR observation ID; (3) best-fit column density, in units of 10 cm ; (4) best-fit photon index; (5) χ over degree of freedom; (6) −12 −1 −2 3–79 keV unabsorbed flux derived from the best-fit spectral model, in units of 10 erg s cm ; (7) 2–10 keV intrinsic luminosity derived from the NuSTAR spectrum; (8) 2–10 keV intrinsic luminosity of the brighter nucleus derived from Chandra data. the Hou et al. (2020) sample, which is consistent with their of which are X-ray detected. The remaining 27 nuclei are original optical classification. When considering the fraction of classified as SF nuclei, but only eight (∼30%) of them are pairs containing at least one X-ray-detected nucleus with X-ray-detected, suggesting that the SF activity does not +7% L > 10 ,we find 32% (30 out of 92 pairs) for the contribute strongly to the observed X-ray emission, at least in 2−10 -7% current sample, which is again somewhat lower than that of the this subset of the sample with optical emission-line measure- +11% ments. We note that only two pairs in our sample have both Hou et al. (2020) AGN sample (47% ; 32 out of 67 pairs). -10% nuclei classified as SF. These and additional detection rates are reported in Table 4. We further use SDSS spectroscopic star-formation rates (SFRs; Brinchmann et al. 2004) provided by the MPA-JHU 4.2. Global X-Ray Properties DR7 catalog to estimate the SF-contributed X-ray luminosity. The left panel of Figure 4 shows L against the hardness 2−10 We note that the SFR is based primarily on the Hα emission ratio of the 70 X-ray-detected nuclei in the close galaxy pairs line, which might be contaminated in the presence of an AGN, (black squares). The hardness ratio, which is defined as but such an effect should lead to an overestimate of the SF- HR = (H − S)/(H + S), is calculated from the observed photon contributed X-ray luminosity, thus strengthening the following flux in the S (0.5–2 keV) and H (2–8 keV) bands using a conclusion. The information of SFR is available for 137 of the Bayesian approach (Park et al. 2006). For the nuclei that are not 184 nuclei in the entire sample. Following Hou et al. (2020), detected in the H band, we show the 3σ upper limit of L by 2−10 we adopt the empirical relation of Ranalli et al. (2003), arrows in the plot. The X-ray counterparts of the AGN pairs (blue circles) and SFG pairs (red triangles) from Hou et al. SFR SF 39 -1 L =´ 4.5 10 erg s , () 1 0.5-2 -1 (2020) are also plotted for comparison (excluding those already M yr included in the new sample). SFR Sixteen nuclei in the current sample are found to have SF 39 -1 L =´ 5.0 10 erg s , () 2 42 −1 21 - 0 -1 L > 10 erg s . These 16 nuclei are probably bona fide M yr 2−10 AGNs, but notably only four of them are found in a pair which has an rms scatter of 0.27 dex and 0.29 dex in the containing another X-ray-detected nucleus (J0907+5203, 0.5–2 keV and 2–10 keV band, respectively. J1058+3144, J1126+3515, and J1414−0000). The majority Figure 5 shows the comparison between the measured X-ray of close galaxy pairs, however, are found at the bottom left 41 −1 luminosity and the empirical X-ray luminosity due to star portion with relatively low luminosities (L < 10 erg s ) 2−10 SF SF formation (L , L ) in the two bands. The majority of the and a negative HR (i.e., a soft spectrum), a region also 0.5-2 21 - 0 detected nuclei lie significantly above the predicted SF- occupied by most SFG pairs. This may suggest that the X-ray contributed luminosity. This holds in both bands, and more emission of these nuclei are dominated by SF activities (e.g., so in the 2–10 keV band. Still, a few SF nuclei (magenta stars) high-mass X-ray binaries and circumnuclear hot gas heated by and a few composite nuclei (orange circles) have their X-ray supernovae) rather than an AGN. However, this does not totally preclude the possibility that some of these sources host luminosity consistent with the predicted SF luminosity. An an accreting SMBH, either intrinsically weak or heavily X-ray AGN is likely absent or heavily obscured in these nuclei. obscured by circumnulcear cold gas with a high column In the meantime, the majority (but all) of the optically classified density. In such a case, the observed soft X-rays probably arise AGN (cyan diamonds) lie significantly above the predicted SF luminosity, indicating that an AGN is indeed powering the further away from the SMBH. observed X-ray emission from these objects. Overall, Figure 5 Seventy-five nuclei in the current sample have reliable 41 −1 suggests that L = 10 erg s can be taken as a practical optical emission-line measurements provided by the MPA-JHU 2−10 threshold above which a genuine AGN is present and SDSS DR7 catalog. For these nuclei, we plot a standard BPT dominates the X-ray emission. Of the 184 nuclei, 32 have diagram (Baldwin et al. 1981), utilizing the line ratios of L above this threshold. Additionally, 83 nuclei have their [O III]/Hβ and [N II]/Hα to provide a canonical diagnosis of 2−10 3σ upper limit above this threshold. We note that only two of their nature, namely, SF, AGN, or SF/AGN composite, as the 92 pairs in the current sample (J0907+5203 and J1414- shown in the right panel of Figure 4. Forty-eight of the 75 0000, shown as green and yellow crosses in Figure 5) have nuclei can be classified as an AGN or composite, the majority both nuclei detected above this threshold. We also test the SF-contributed X-ray luminosity using https://wwwmpa.mpa-garching.mpg.de/SDSS/DR7/ different relations provided in Lehmer et al. (2010), Mineo 10 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Figure 3. SDSS gri-color composite (first and third columns) and Chandra/ACIS 0.5–8 keV (second and fourth columns) images of the eight newly-found close galaxy pairs with both nuclei detected in the X-rays. Each panel has a size of 50″ × 50″ unless otherwise labeled. North is up and east is to the left. Magenta circles denote positions of the optical nuclei. Green circles represent the 90% ECR of local PSF. et al. (2012), and Fragos et al. (2013). The detailed calculation substantially higher. To check the possibility of a buried but and figures are presented in Appendix. The overall distributions intrinsically luminous AGN, we examine the infrared (IR) color are very similar to that derived in Figure 5, which help to of each galaxy pair provided by the Wide-field Infrared Survey confirm AGNs dominate the X-ray emission for nuclei with Explorer (WISE) survey Wright et al. (2010). Specifically, we 41 −1 L = 10 erg s . 2−10 adopt the color of W1 (3.4 μm) − W2 (4.6 μm), which is sensitive to the presence of a luminous AGN (Jarrett et al. 4.3. Obscured AGNs Probed by WISE Color and NuSTAR 2011; Stern et al. 2012; Satyapal et al. 2014). Figure 6 plots Spectra L versus W1 − W2, for both the close galaxy pairs and 2−10 AGN pairs. Given the relatively large WISE PSF The 2–10 keV luminosity, which is derived by assuming a 22 −2 (FWHM ≈ 6″), the two nuclei in many of these pairs are moderate absorption column density N = 10 cm , might be underestimated if the true absorption column density were unresolved and thus share the same value. Nevertheless, this 11 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 4 Comparison of X-Ray Detection Rates Sample Sample Size Detection Requirement No. of Detections Detection Rate (1)(2)(3)(4)(5) +3% Nuclei in close pairs 184 log L > 41 32 18% 2−10 -3% −4 +4% Nuclei with M 91 L /L > 10 14 15% BH 2−10 Edd -5% +5% Nuclei in H20 AGN pairs (all) 134 log L > 41 36 27% 2−10 -5% +10% Nuclei in H20 AGN pairs (r  20 kpc) 56 log L > 41 22 39% p 2−10 -10% +7% Close pairs 92 at least one detection and log L > 41 30 32% 2−10 -7% +2% Close pairs 92 dual detection and log L > 41 2 2% 2−10 -2% +5% Close pairs 92 at least one detection and log L > 42 16 17% 2−10 -5% +12% Close pairs (r < 10 kpc) 40 at least one detection and log L > 41 17 41% p 2−10 -13% +8% Close pairs (r > 10 kpc) 52 at least one detection and log L > 41 13 25% p 2−10 -8% +11% H20 AGN pairs (all) 67 at least one detection and log L > 41 32 47% 2−10 -10% +17% H20 AGN pairs (r  20 kpc) 28 at least one detection and log L > 41 19 66% p 2−10 -18% −4 +13% Close pairs both with M 26 at least one detection and L /L > 10 9 33% BH 2−10 Edd -15% Note. Quoted errors, at 1σ, take into account the Poisson error associated with both the umerator and denominator. does not significantly affect our following conclusion, because galaxies. Following Hou et al. (2020), we bin the data points a luminous AGN, when existed, is expected to dominate the (including the upper limits) into several intervals of r and WISE flux. estimate the mean luminosity of each r bin using the Figure 6 shows that most nuclei fall on the blue side of Astronomy SURVial Analysis (ASURV; Feigelson & Nel- W1 − W2 = 0.5, an empirical threshold that separates star- son 1985), a maximum likelihood estimator of the statistical forming galaxies from AGNs (Satyapal et al. 2014). On the properties of censored data, as is the case here. We have chosen 43 −1 other hand, nearly all nuclei with L > 10 erg s have even bins in logarithmic space covering 3.0 kpc „ r „ 20 kpc 2−10 p W1 − W2 > 0.5, finding good agreement between the X-ray and ensured that each bin contain at least 10 nuclei to minimize and IR AGN classifications. A curious exception is the nucleus random fluctuation. We note that the main conclusion below is (J112648.50+351503.2) with the highest L insensitive to the exact choice of bins. The resultant mean 2−10 43 −1 (1.0 × 10 erg s ), which has W1 − W2 ∼ 0.37, but its high 2–10 keV luminosity of the close galaxy sample is shown by X-ray luminosity warrants an AGN classification, This nucleus large black squares. For comparison, the full AGN pair sample is likely accompanied by intense IR starlight of the host galaxy. of Hou et al. (2020) is shown by blue circles, which covers a Also remarkable are a handful of nuclei with W1 − W2 > 0.5 wider range of r up to 100 kpc. The mean 2–10 keV 43 −1 but also with L  10 erg s . Some of these nuclei might luminosities of optically selected single AGNs and SFG pairs, 2−10 host a heavily obscured AGN and have their L significantly taken from Hou et al. (2020) and calculated with ASURV, are 2−10 underestimated. Fortunately, five of these nuclei have an also plotted for comparison (green and red horizontal lines). available NuSTAR spectrum (Section 3.3). In four of the five The two outermost bins (10 kpc  r < 20 kpc) have a mean cases (J0841+0102, J1338+4816, J1354+1238, and J1450 L comparable with each other within the statistical 2−10 +0507), the 2–10 keV luminosity converted from the best-fit uncertainty, which is also comparable to that of optically 41 −1 model to the NuSTAR spectrum is actually two to seven times selected single AGNs (2.6[ ± 0.6] × 10 erg s ). This sug- higher than the default value of L derived from the gests that galaxy interactions have not generally boosted the 2−10 Chandra data (Table 3; marked by magenta stars in Figure 6). AGN activity at such intermediate separations, if the mean In the remaining case (J1125+5423), the NuSTAR spectrum- X-ray luminosity of single AGNs can be taken as the reference based luminosity is actually two times lower, which might level. On the other hand, as noted by Hou et al. (2020) and reflect intrinsic variability. Nevertheless, the absorption column reiterated here, the AGN pairs at similar r show a substantially densities inferred from the NuSTAR spectra are generally higher mean L . This difference might again be understood 2−10 23 −2 moderate, and in all cases lower than 2 × 10 cm (Table 3). as a systematically higher fraction of luminous AGNs in the This suggests that the L in the other nuclei with Hou et al. (2020) sample, which pertains to their optical 2−10 W1 − W2 > 0.5 but without NuSTAR observations are rather classification. We note that a handful of nuclei with the lowest unlikely to have been underestimated by more than a factor L have a value (or upper limit) consistent with the mean of 2−10 40 −1 of 10. SFG pairs (1.3[ ± 0.3] × 10 erg s ), indicating that an AGN is intrinsically weak or absent in these nuclei. At smaller r , the mean L finds its highest value at the p 2−10 4.4. Mean X-Ray Luminosity versus Projected Separation third bin (6.3 kpc < r < 9.0 kpc), which is about an order of The left panel of Figure 7 shows L (or upper limits for magnitude higher than the mean of the two outer bins as well as 2−10 non-detected nuclei) as a function of projected separation r for the mean of single AGNs. The mean L of the second bin is p 2−10 the close galaxy pairs. As mentioned in Section 1, r is taken as also significantly elevated. This might be understood as a sign a proxy for the merger phase, with the smallest r (a few of enhanced SMBH accretion due to merger-driven gas kiloparsecs) indicating the late stage of a merge. A substantial inflows. However, it is noteworthy that the four nuclei with 43 −1 scatter in L over nearly five orders of magnitude exists in the highest luminosities (L  10 erg s ) were targeted 2−10 2−10 this plot, reflecting a wide range of AGN activity in these by Chandra because they were known to be luminous in either 12 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Figure 4. Left: 2–10 keV luminosity vs. hardness ratio. The black squares, blue circles, and red triangles represent X-ray counterparts of close galaxy pairs (current sample), AGN pairs, and SFG pairs (Hou et al. 2020), respectively. Those nuclei undetected in the hard band are marked by arrows. Right: Standard BPT diagram for the nuclei which have reliable optical emission-line measurements (i.e., with a S/N > 3 in each of the four lines). The solid and dashed lines, taken from Kewley et al. (2001) and Kauffmann et al. (2003),define the canonical regions occupied by star-forming nuclei, composite nuclei and AGNs, which are marked by the cyan diamonds, orange circles, and magenta stars, respectively (same in the left panel). Filled and open symbols represent X-ray detected and non-detected nuclei, respectively. Figure 5. 0.5–2 keV (left panel) and 2–10 keV (right panel) luminosity vs. the predicted luminosity due to star-formation activity. The black squares represent X-ray counterparts of the close galaxy pairs. Those nuclei undetected in a given band are marked by arrows. The black solid line indicates a 1:1 relation, with the pair of dashed lines representing the rms scatter (Equations (1) and (2)). The cyan diamonds, magenta stars, and orange circles denote the optically classified AGNs, SF 41 −1 nuclei, and composite nuclei, respectively. The two pairs with both nuclei detected above L = 10 erg s are labeled as green and yellow crosses in the right 2−10 panel. hard X-rays or the IR, which potentially introduces a selection mean value of single AGNs. Overall, L –r relation 2−10 p effect. We find that removing these nuclei from the second and suggests little evidence for merger-induced AGN activity in third r bins results in a mean L much closer to the mean of close galaxy pairs. p 2−10 This is reinforced when the absolute X-ray luminosity is single AGNs. Therefore it remains inclusive whether the upward rising trend between the fourth and third bins is replaced by the X-ray Eddington ratio (L /L ), as shown 2−10 Edd in the right panel of Figure 7. Here L is the Eddington intrinsic. More surprisingly, the mean L continues to Edd 2−10 decrease toward the smallest r . Nine of the 10 nuclei in the luminosity, which scales with an estimated black hole mass (M ) based on the stellar velocity dispersion (σ ) from the innermost bin, in fact, have L (or upper limit) below the BH 2−10 13 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. only four have a 2–10 keV unabsorbed luminosity 43 −1 10 erg s , a conventional threshold for luminous AGNs. Nevertheless, the majority of the nuclei have an X-ray luminosity (or an upper limit in the case of non-detection) significantly above the empirical luminosity due to star- forming activity (Figure 5). This suggests that a weakly accreting SMBH, rather than star formation, is responsible for the observed X-ray emission in most nuclei. Optical line ratios, which are available for 75 nuclei, support this view (Figure 4). We examine whether the X-ray-detection/non-detections are related to the host galaxy properties. By comparing the distributions of redshift, r-band absolute magnitude of the host galaxy, and X-ray detection limit, between the detected and non-detected nuclei, we find that none of these parameters is statistically distinct between the detected and non-detected nuclei. Our visual examination also does not reveal systematic differences in the global morphology (e.g., more disk- dominated) between the detected and undetected subsets. In Figure 8, we further compare the distributions of stellar velocity dispersion (left panel) and SFR (right panel) between the X-ray detected (red histogram) and undetected nuclei (black histogram). The two subsets both show a large scatter in their Figure 6. 2–10 keV luminosity vs. W1 − W2 color. The close galaxy pairs and AGN pairs of Hou et al. (2020) are shown by black squares and blue circles, stellar velocity dispersion, but there is no systematic difference respectively. The solid and open symbols represent detections and non- between the two. This indicates that the detected nuclei are not detections. Error bars are neglected for clarity. The magenta stars mark the preferentially found in galaxies with a more massive SMBH 2–10 keV luminosity derived from NuSTAR spectra, which are available for (assuming that M is statistically reflected by the stellar five pairs. BH velocity dispersion). On the other hand, a larger fraction of high −1 SFR (0.1 M yr ) is found with the detected subset, MPA-JHU catalog and the empirical M –σ relation from BH although, as previously noted, the presence of an AGN may Gültekin et al. (2009). To ensure a reasonable estimate of M , BH cause an overestimate of the SFR. Neglecting this caveat, such we have discarded those nuclei with values lower than 10 M a trend might be taken as evidence that a larger amount of fuel or higher than 10% of the host galaxy mass. In total, 91 nuclei is available in the detected subset for both star formation and have a reliable M and appear in the L /L –r plot. We BH 2−10 Edd p 43 −1 SMBH accretion. note that two of the four nuclei with L > 10 erg s are 2−10 We also examine the relation between stellar mass ratio of thus not included. The mean L /L of the Hou et al. 2−10 Edd the pairs and the observed X-ray luminosity. Only about half (2020) AGN pairs are plotted for comparison, as well as the galaxy pairs (45/92) have reliable stellar mass measurement for mean L /L of the single AGNs derived in a similar way. 2−10 Edd both nuclei. Among them, only 27 galaxy pairs have at least Clearly, the mean L /L of the close galaxy pairs shows 2−10 Edd one X-ray detected nucleus, which is only a small fraction no significant enhancement relative to that of single AGNs at any r bin. compared to the whole sample. There is a tentative trend that the more massive galaxy in a pair is more likely to host a more luminous AGN. 5. Summary and Discussion Since essentially all nuclei have an SF-contributed luminos- 41 −1 ity below L = 10 erg s (Figure 5), it is practical to 2−10 In this work, we have presented the detection and statistical adopt this as the threshold, above which a genuine AGN can be analyses of X-ray nuclei in a newly compiled sample of 92 identified. This allows us to derive the fraction of pairs close galaxy pairs at low redshift (z ¯ ~ 0.07), based on archival containing at least one X-ray-detected nucleus (the case of only Chandra observations. The sample is designed to have one detected nucleus is sometimes referred to as an “offset projected separations 20 kpc and thus representative of the AGN”), which is ∼33% (Section 4.1). Raising the threshold to intermediate-to-late stage of galaxy mergers. Also by design, 42 −1 10 erg s or restricting to dual AGNs (i.e., both nuclei the sample requires no optical emission-line classification of +5% +2% detected) results in a fraction of 17% and 2% , the nuclei, thus it is largely (but not completely) free of -5% -2% respectively. These may serve as a useful point of reference selection bias for or against intrinsic AGN activity. This sample for theoretical and numerical studies of AGN triggering in has similar X-ray detection sensitivity (down to a limiting 40 −1 interacting galaxy pairs, by virtue of our sample being largely luminosity of ∼10 erg s ), redshift, and host galaxy mass unbiased to AGN selection. Applying different definitions of (Figure 1) compared to the close AGN pairs studied by Hou AGNs (e.g., based on a threshold of bolometric luminosity, et al. (2020), but is a factor of about two larger in size, helping X-ray luminosity, or Eddington ratio), existing numerical to relieve concern about a small number statistics. These factors studies, including both idealized galaxy merger simulations together offer an unprecedented opportunity for probing the (e.g., Capelo et al. 2015; Capelo & Dotti 2017; Solanes et al. connection between galaxy interaction and AGN activity through nuclear X-ray emission, which is generally thought 2019) and cosmological simulations (e.g., EAGLE, Rosas- to be a robust diagnostic of AGNs. Guevara et al. 2019; ASTRID, Chen et al. 2023), typically Despite the excellent sensitivity achieved, less than half of predict a dual-AGN fraction of few percent for luminous AGNs the 184 nuclei are firmly detected in the X-rays, among which or accretion rates close to the Eddington limit. This is 14 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Figure 7. Left: 2–10 keV luminosity as a function of projected separation. The small black squares and blue circles represent close galaxy pairs and the Hou et al. (2020) AGN pairs, respectively. The 3σ upper limit of undetected nuclei are shown by arrows. For each r bin, the mean luminosities of close galaxy pairs and the Hou et al. (2020) AGN pairs are represented by the large black squares and blue circles, respectively. The mean value of single AGNs (star-forming galaxy pairs) from Hou et al. (2020) is shown by the green (red) horizontal solid line, with 1σ error bars represented by the dashed green (red) lines. Right: Similar to the left panel, but for the X-ray Eddington ratio. The same r bins as in the left panel are adopted. Only those nuclei with a reliable black hole mass estimate (Table 1) are included. Figure 8. Stellar velocity dispersion (left panel) and star-formation rate (right panel) distributions of the X-ray detected (red histogram) and undetected nuclei (black histogram). The vertical lines mark the median values. compatible with the above statistics. However, it is noteworthy further confirmed with the current sample of close galaxy pairs, that current simulations still lack the ability of self-consistently although one should bear in mind that the innermost bins are driven determining the accretion rate and the accretion-induced X-ray by a relatively small number of nuclei. Indeed the mean luminosity luminosity, owing primarily to the lack of resolutions down to of the innermost r bin is fully consistent with the mean of optically the sphere of gravitational influence of the SMBH. This is classified single AGNs. The fraction of nuclei with 41 −1 further complicated by the uncertain degree of circumnulear L > 10 erg s ,18%± 3% (Table 4), is even marginally 2−10 obscuration. Hence caution is warranted when comparing the lower than that of the single AGNs (24%± 5%; Hou et al. 2020). observed and predicted AGN fractions. At face value, this suggests that close galaxy interactions do not Hou et al. (2020) revealed a rather surprising trend of decreasing effectively result in boosted AGN activity, which is contradictive mean X-ray luminosity with decreasing projected separation in their with the general prediction of the aforementioned numerical AGN pairs with r  10 kpc. This is reproduced in Figure 7 and simulations, in which tidal torques become stronger at the smaller 15 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. separations and thus more effective in driving gas to the vicinity of may not occur until shortly after the second pericentric passage, the SMBH. Interestingly, a recent study by Jin et al. (2021) based which lasts for a few tens of megayears (Capelo et al. 2015).At and after this stage, the separation of the two nuclei remain at on SDSS/MaNGA integral-field spectroscopic mapping of low-z no more than 10 kpc, which is consistent with the inner bins in galaxies, also found no significant excess in the AGN fraction in Figure 7.An efficient feedback can explain the moderate any merger phase compared to that of isolated galaxies. The galaxy column densities inferred for at least a subset of the nuclei. The pair sample of Jin et al. (2021) is free of pre-selection of AGN feedback is likely in a kinetic mode mediated by jets and winds characteristics, which is similar to ours. (Yuan & Narayan 2014), given that most nuclei have a low Two physical scenarios were proposed by Hou et al. (2020) Eddington ratio (Figure 7). Future high-resolution radio and to explain the behavior revealed in Figure 7, which we optical spectroscopic observations will be crucial to search for elaborate here. The first is an obscuration effect. In close galaxy direct evidence of this feedback in the close galaxy pairs. pairs, gravitational perturbation can be sufficiently strong to induce gas inflows in one or both galaxies, which in turn result M.H. is supported by the National Natural Science in the accumulation of circumnuclear cold gas that heavily Foundation of China (12203001) and National Postdoctoral obscures even the hard X-rays, regardless of the intrinsic AGN Program for Innovative Talents of China Postdoctoral Science luminosity. Indeed, observational evidence has been gathered Foundation (grant BX2021016). H.L. and Z.L. acknowledge for heavily obscured AGN pairs at kiloparsec separations support by the National Natural Science Foundation of China (Satyapal et al. 2017; Pfeifle et al. 2019; De Rosa et al. 2023). (12225302). S.F. acknowledges support from National Natural However, obscuration cannot be the sole cause of the low-to- Science Foundation of China (No. 12103017) and Natural moderate luminosities observed in most nuclei of our sample, Science Foundation of Hebei Province (No. A2021205001).X. in view of the following countering evidences. On the one L. acknowledges support from NSF grants AST-2108162 and hand, in the five nuclei with a high-quality NuSTAR spectrum, AST-2206499. The authors wish to thank Drs. Yanmei Chen the best-fit foreground absorption column densities (Table 3) and Zongnan Li for helpful discussions. 24 −2 are far below that required (10 cm ) to completely block X-ray photons below a few kilo electron volts. On the other Appendix hand, in a recent attempt of directly detecting circumnuclear Comparison of Star Formation Contribution to X-Ray cold gas in seven pairs of dual-AGNs based on high-resolution Luminosity CO observations, Hou et al. (2023) found no evidence for an 24 −2 Lehmer et al. (2010) calibrated the 2–10 keV X-ray emission equivalent hydrogen column density 10 cm in any of the from both high- and low-mass X-ray binaries (HMXBs and 14 nuclei, which are all included in Hou et al. (2020;10 LMXBs) based on a sample of 17 luminous infrared galaxies included in the current sample). Nevertheless, it remains and presented an empirical correlation between 2 to 10 keV interesting to see whether a dense circumnuclear gas exists in gal luminosity L , SFR, and stellar mass as the several nuclei with the smallest r (5 kpc), which also HX have the lowest apparent L , through higher resolution CO 2−10 gal LM=+abSFR, () A1 HX observations and hard X-ray observations. In the second scenario, most SMBHs in the close galaxy 28 −1 -1 where α = (9.05 ± 0.37) × 10 erg s M and pairs are currently weakly accreting, which is the result of 39 −1 −1 −1 β = (1.62 ± 0.22) × 10 erg s (M yr ) . As estimated negative AGN feedback that have expelled the circumnuclear based on SDSS images, we adopted a uniform factor of 20% gas and prevents the SMBH from maintaining a high level of for the contribution from LMXBs to enclose the stellar mass in accretion. Numerical simulations of idealized galaxy mergers the nuclear region (∼2″). Since only half of the galaxies have suggest that gas inflows may start as soon as the first pericentric stellar mass measurement, the data points are reduced passage of the two galaxies, typically at a physical separation of 10 kpc, while substantial enhancement of SMBH accretion compared to the others (Figure 9, left panel). This relation Figure 9. 2–10 keV luminosity vs. the predicted luminosity due to star-formation activity according to the relation in Lehmer et al. (2010; left panel), Mineo et al. (2012; middle panel), and Fragos et al. (2013; right panel), respectively. The black squares represent X-ray counterparts of the close galaxy pairs. 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(2012) considers X-ray emission only from Feigelson, E. D., & Nelson, P. I. 1985, ApJ, 293, 192 HMXBs with the contamination from LMXBs carefully Feng, S., Shen, S.-Y., Yuan, F.-T., et al. 2019, ApJ, 880, 114 subtracted based on a sample of 29 nearby star-forming Foord, A., Gültekin, K., Nevin, R., et al. 2020, ApJ, 892, 29 galaxies. But the predicted SF-contributed luminosity is given Fragos, T., Lehmer, B., Tremmel, M., et al. 2013, ApJ, 764, 41 Fu, H., Myers, A. D., Djorgovski, S. G., et al. 2015a, ApJ, 799, 72 in 0.5–8 keV as Fu, H., Wrobel, J. M., Myers, A. D., Djorgovski, S. G., & Yan, L. 2015b, XRB -- 139 1 ApJL, 815, L6 LM () erg s =´ 2.61 10 SFR( yr ). (A2) 0.5-8 keV Gross, A. C., Fu, H., Myers, A. D., Wrobel, J. M., & Djorgovski, S. G. 2019, So we multiply a conversion factor to calculate the 2–10 keV ApJ, 883, 50 Gültekin, K., Cackett, E. M., Miller, J. M., et al. 2009, ApJ, 706, 404 luminosity. 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A Chandra X-Ray Survey of Optically Selected Close Galaxy Pairs: Unexpectedly Low Occupation of Active Galactic Nuclei

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IOP Publishing
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© 2023. The Author(s). Published by the American Astronomical Society.
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0004-637X
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1538-4357
DOI
10.3847/1538-4357/acc5e1
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Abstract

High-resolution X-ray observations offer a unique tool for probing the still-elusive connection between galaxy mergers and active galactic nuclei (AGNs). We present an analysis of nuclear X-ray emission in an optically selected sample of 92 close galaxy pairs (with projected separations 20 kpc and line-of-sight velocity offsets −1 <500 km s ) at low redshift (z ¯ ~ 0.07), based on archival Chandra observations. The parent sample of galaxy pairs is constructed without imposing an optical classification of nuclear activity, thus it is largely free of selection effect for or against the presence of an AGN. Nor is this sample biased for or against gas-rich mergers. An X-ray +5% source is detected in 70 of the 184 nuclei, giving a detection rate of 38% , down to a 0.5–8 keV limiting -5% 40 −1 luminosity of 10 erg s . The detected and undetected nuclei show no systematic difference in their host galaxy properties such as galaxy morphology, stellar mass, and stellar velocity dispersion. When potential contamination 41 −1 +3% from star formation is avoided (i.e., L > 10 erg s ), the detection rate becomes 18% (32/184), which 2−10 keV -3% shows no excess compared to the X-ray detection rate of a comparison sample of optically classified single AGNs. +2% The fraction of pairs containing dual AGN is only 2% . Moreover, most nuclei at the smallest projected -2% separations probed by our sample (a few kiloparsecs) have an unexpectedly low apparent X-ray luminosity and Eddington ratio, which cannot be solely explained by circumnuclear obscuration. These findings suggest that close galaxy interaction is not a sufficient condition for triggering a high level of AGN activity. Unified Astronomy Thesaurus concepts: Galaxy nuclei (609); Interacting galaxies (802); Galaxy mergers (608); X- ray active galactic nuclei (2035) Supporting material: machine-readable tables 1. Introduction can significantly grow its mass, preceding the formation of an SMBH binary and their ultimate merger (Merritt & It is a generic prediction of the standard paradigm of Milosavljević 2005). hierarchical structure formation that most galaxies frequently As observational validation of the above scenario, a number interact with other galaxies during their lifetime. When the two of systematic searches for dual AGN candidates have been interacting galaxies are gravitationally bound, their ultimate conducted over the past decade, primarily in the optical band, fate is to merge, eventually forming a more massive galaxy. In thanks to wide-field, homogeneous spectroscopic surveys such the course of galaxy mergers, tidal force and ram pressure act as the Sloan Digital Sky Survey (SDSS). In particular, the to significantly redistribute the stellar and gaseous contents of search for galactic nuclei with double-peaked narrow emission the interacting pair (Toomre & Toomre 1972; Barnes & lines (e.g., [O III]; Wang et al. 2009; Liu et al. 2010) aims at Hernquist 1992). It is theoretically predicted and has been tight AGN pairs (typically 1–10 kpc in separation, but even demonstrated by numerical simulations (e.g., Di Matteo et al. less) that pertain to the late stage of merger, whereas the search 2005) that upon close passages, gravitational torques drive gas for resolved pairs of galactic nuclei both showing the optical inflows to the center of one or both galaxies, potentially emission-line characteristics of Seyfert or Low Ionization triggering nuclear star formation and active galactic nuclei Nuclear Emission-line Region (LINER) covers larger projected (AGNs). A physical consequence of this scenario is the separations up to ∼100 kpc (Liu et al. 2011, hereafter L11). prevalence of AGN pairs in (major) galaxy mergers, which Confirmation of the AGN nature in these optically selected involve two SMBHs with simultaneous active accretion. candidates, however, often require follow-up observations in Specifically, “dual AGNs,” AGN pairs with a separation the X-ray and/or radio bands (Comerford et al. 2011; 10 kpc in projection, are generally expected at the inter- Silverman et al. 2011; Teng et al. 2012; Liu et al. 2013;Fu mediate-to-late stage of mergers (see recent review by De Rosa et al. 2015a, 2015b; Brightman et al. 2018; Gross et al. 2019; et al. 2019). This is a crucial phase during which the SMBH(s) Hou et al. 2019; Foord et al. 2020), which are generally thought to trace immediate radiation from the SMBH (more precisely, Original content from this work may be used under the terms from the accretion disk, corona, and/or jets) and tend to be of the Creative Commons Attribution 4.0 licence. Any further more immune to circumnuclear obscuration. Infrared observa- distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. tions have also played an effective role in revealing dual 1 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. AGNs, especially in gas-rich merging systems, which tend to pre-selection of optical AGN characteristics as applied in Hou select highly obscured AGNs (Satyapal et al. 2014, 2017; et al. (2020), thus allowing for an unbiased view of AGN activity through their nuclear X-ray emission. This paper is Pfeifle et al. 2019). An alternative approach (Koss et al. 2012) structured as follows. Section 2 describes the construction of a starts with a hard X-ray (10 keV) AGN detected in the Swift/ new sample of close galaxy pairs with archival Chandra BAT survey and tries to associate it with another AGN in a observations. Data analysis toward detection and characteriza- companion galaxy within a projected distance of 100 kpc, if it tion of the nuclei are detailed in Section 3. Section 4 presents exists. This approach, however, inevitably introduced selection the results, including the properties and statistics of the X-ray bias toward X-ray-luminous AGNs due to the moderate detected nuclei and a reexamination of the behavior of L as sensitivity of Swift/BAT. Nevertheless, these approaches have 2−10 a function of r . Section 5 summarizes the study and address achieved a certain degree of success, revealing a growing p most significant implications. Throughout this work, we number of AGN pairs and dual AGNs. assume a concordance cosmology with Ω = 0.3, Ω = 0.7, Clearly, having a sizable and unbiased sample of genuine m Λ −1 −1 and H = 70 km s Mpc . Errors are quoted at 1σ confidence dual AGNs is crucial for a thorough understanding of the 0 level unless otherwise stated. causality between galaxy mergers and AGN triggering. Recently, Hou et al. (2020, hereafter H20) carried out a systematic search for X-ray-emitting AGN pairs, using archival 2. The Sample of Close Galaxy Pairs Chandra observations and based on the Liu et al. (2011) sample In this work, we construct a new sample of close galaxy of ∼10 optically selected AGN pairs at low redshift (with a pairs based on the parent sample of galaxy pairs recently median redshift z ~ 0.1). Thanks to the superb angular presented by Feng et al. (2019, hereafter F19). The F19 sample resolution of Chandra, unattainable from any other X-ray itself was extracted from the SDSS DR7 (Abazajian et al. 2009) facility, one can unambiguously resolve and localize the photometric galaxy catalog, with ∼95% of the cataloged putative AGN even in close pairs. More importantly, the typical galaxies having an available spectroscopic redshift, which was sensitivity of Chandra observations used by Hou et al. (2020) is primarily from SDSS and supplemented by LAMOST (Luo sufficient to probe low-luminosity AGNs (i.e., weakly accreting et al. 2015; Shen et al. 2016), GAMA (Baldry et al. 2018), and SMBHs) down to a limiting 2–10 keV X-ray luminosity of 40 −1 other spectroscopic surveys (see detailed description in Feng L ∼ 10 erg s , which is necessary for a complete census 2−10 et al. 2019). A close galaxy pair was selected if the two of nuclear activity. member galaxies have a line-of-sight velocity offset Among 67 pairs of the optically selected AGN candidates −1 Δv < 500 km s and a projected separation r  20 kpc. We with useful Chandra data, Hou et al. (2020) found that 21 pairs also required that each galaxy has only one neighbor galaxy show significant X-ray emission from both nuclei (i.e., with a similar redshift within a projected separation of 100 kpc probable AGN pairs), with an additional 36 pairs having only −1 and a velocity offset of 500 km s , to minimize environmental one of the two nuclei detected. The X-ray detection rate of all effects typical of compact groups or clusters. Contrary to Feng 134 nuclei, 58% ± 7% (1σ Poisson errors), is significantly −1 et al. (2019), who focused on pairs with r > 10h kpc, we higher than that (17% ± 4%) of a comparison sample of star- impose no lower limit on r . However, due to the resolution forming galaxy pairs, classified also based on optical emission- limit of the optical surveys (∼1″), the Feng et al. (2019) sample line ratios. Moreover, interesting trends were revealed for the still suffers from incompleteness for the most closely separated mean X-ray luminosity as a function of the projected pairs (i.e., 1 kpc). separation, r , which is taken as a proxy for the merger phase, We thus have a preliminary list of 3337 close galaxy pairs. A where larger (smaller) r represents the earlier (later) stage of a comparison with the Liu et al. (2011) sample of optically selected merger. First, L increases with decreasing projected 2−10 AGN pairs shows that the two samples have 130 common pairs, separation in AGN pairs at r  20 kpc, suggesting enhanced whereas 3207 pairs are in the Feng et al. (2019) sample but not in SMBH accretion even in early-stage mergers, perhaps related the Liu et al. (2011) sample. This difference partly stems from the to the first pericentric passage of the two galaxies. Second and fact that the Liu et al. (2011) sample, which was primarily based unexpectedly, L decreases (rather than increases) with 2−10 on SDSS DR7 spectroscopic redshifts, suffers from the restriction decreasing r at r  10 kpc, which appears contradicting with p p of SDSS fiber collision and thus is missing closely separated the intuitive expectation that tidal-force-driven gas inflows galaxy pairs. The Feng et al. (2019) sample was exactly designed become more and more prevalent as mergers proceed. Despite to overcome this incompleteness, thereby significantly increasing the small number statistics, Hou et al. (2020) proposed two the number of close galaxy pairs. Moreover, the Liu et al. (2011) physical explanations for this latter behavior: (i) merger- sample required both galaxies in a pair to have a Seyfert or induced gas inflows become so strong that an enhanced central LINER classification based on the optical emission-line diag- concentration of cold gas heavily obscures even the hard (2–10 nostics, whereas the Feng et al. (2019) sample only required a keV) X-rays; (ii) AGN feedback triggered by the first spectroscopic redshift based primarily on stellar continuum, thus pericentric passage acts to expel gas from the nuclear region it, in principle, minimizes the selection bias for or against AGN and consequently suppress or even halt SMBH accretion. The activity in closely interacting galaxies (though see Section 4.4 for latter possibility is of particular interest, potentially offering potential bias for the most luminous AGNs in a few Chandra insight into the still-elusive processes of SMBH feeding and observations), as well as selection bias for or against gas-rich feedback during an indispensable stage of galaxy evolution. mergers. Extending the study of Hou et al. (2020), in this work we use We cross-matched the Feng et al. (2019) sample with the archival Chandra observations to survey the nuclear X-ray Chandra X-ray data archive to select pairs with observations emission from a new sample of close galaxies pairs. These taken with the Advanced CCD Imaging Spectrometer (ACIS) close galaxies pairs are selected from optical spectroscopic and publicly available as of June 2022. Similar to Hou et al. surveys (see Section 2 for details), but they are not subject to a (2020), we requested that both galactic nuclei in a pair fall 2 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Figure 1. Redshift (left panel), r-band absolute magnitude (middle panel), and projected angular separation (right panel) distributions of the close galaxy pairs studied in this work (black solid histogram), in comparison with the close AGN pairs (blue dashed) in Hou et al. (2020). The vertical lines mark the median value of the individual samples. within the ACIS field of view and within 8¢ from the aimpoint, while the close AGN pairs have a similar z = 0.062 to ensure the feasibility of source detection and photometry. and M =-21.4 mag. We further visually inspected the SDSS to filter several spurious galaxy pairs, which are most likely compact star- 3. Data Analysis forming clusters/complexes that mimicked a second galactic 3.1. Chandra Data Preparation nucleus. Our final sample consists of 92 optically and X-ray selected galaxy pairs, which have r ranging from 3.0 to The Chandra/ACIS data were reprocessed following the 19.7 kpc. This small fraction (92/3337) reflects the empirical standard procedure, using CIAO v4.13 with the calibration files rule that on average only a few percent of randomly selected CALDB v4.9.5. Among the 92 galaxy pairs in the current sky targets would fall on a Chandra/ACIS footprint. Basic sample, 78 pairs have only one observation, while the the other information of these galaxy pairs are given in Table 1. 14 pairs have been observed more than one time, for which we Our sample is an extension of the AGN pairs and SFG pairs combined all available observations. studied by Hou et al. (2020). The Hou et al. (2020) AGN pairs, Following the procedures in Hou et al. (2020), for each selected from the parent sample of Liu et al. (2011), cover a observation we produced counts, exposure, and point-spread wider range of projected separations (r < 100 kpc) and have function (PSF) maps on the natal pixel scale of 0 492 in the both nuclei classified as an AGN based on the optical emission- 0.5–2 (S),2–8 (H), and 0.5–8 (F) keV band. The exposure line diagnostics. Hou et al. (2020) also constructed a maps and the PSF maps were weighted by a fiducial incident comparison sample of SFG pairs (i.e., both nuclei having the spectrum, which is an absorbed power-law with a photon index optical emission-line diagnostics of star formation). Consider- of 1.7 (a median value for AGN, see Winter et al. 2009) and 22 −2 ing only the close pairs (i.e., those with r < 20 kpc) in Hou absorption column densities N = 10 cm for the H band 21 −2 et al. (2020), there are 28 AGN pairs and 12 SFG pairs. For and N = 10 cm for the S band. clarity, hereafter we refer to AGN pairs or SFG pairs of Hou For targets with multiple observations, the counts, exposure, et al. (2020) as those pairs with r < 20 kpc only, unless and PSF maps of individual observations were reprojected to a otherwise stated. With our new sample, which presumes no common tangential point after calibrating their relative distinction between optically classified AGN and SFG, the total astrometry, to produce combined images that maximize the number of close galaxy pairs with both Chandra and optical source detection sensitivity. Only the I0, I1, I2, and I3 CCDs spectroscopic observations is now more than doubled. We note for the ACIS-I observations and the S2 and S3 CCDs for the that the new sample includes 17 AGN pairs and 1 SFG pair in ACIS-S observations were included at this step. We have Hou et al. (2020). These pairs are kept in the following examined the light curves of each observation and filtered time analysis, but caution is taken not to double-count them when an intervals contaminated by significant particle flares, if any. The analysis also involves those pairs from Hou et al. (2020). There effective exposure time of each target pair ranged from 1.1 to also existsome pairs that belong to Hou et al. (2020) but are 240.1 ks, with a median value of 13.7 ks. not included in the new sample. This is mainly due to the fact that the Liu et al. (2011) sample did not impose the requirement 3.2. X-Ray Counterparts and Photometry on the absence of a third galaxy within 100 kpc and also included some pairs that are not part of the parent galaxy We followed the procedures detailed in Hou et al. (2020) to sample of Feng et al. (2019). search for X-ray counterparts of the optical nuclei in our close Figure 1 compares the redshift (left panel) and SDSS r-band galaxy pairs. We first performed source detection in the 0.5–2, absolute magnitude (M ; middle panel) distributions of the 2–8, and 0.5–8 keV bands for each galaxy pair using the CIAO current sample with those of the AGN pairs in Hou et al. tool wavdetect, with the 50% enclosed count fraction (ECF) (2020). The current sample has a median redshift z ¯ = 0.067 and a median r-band absolute magnitude M =-21.1 mag, http://cxc.harvard.edu/ciao/ 3 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 1 Information of Close Galaxy Pairs with Chandra Observation Name R.A. Decl. zr log M SFR log M log L Flag p * BH X,lim (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) +3.04 J102700.40+174901.0 156.75167 17.81694 0.0665 3.0 10.9 1.09 7.4 40.17 0 -0.97 +29.44 J102700.56+174900.3 156.75233 17.81675 0.0666 3.0 L 12.86 L 40.17 1 -9.38 +8.31 J085837.53+182221.6 134.65637 18.37267 0.0587 3.3 10.4 3.55 7.5 40.39 1 -2.62 +9.05 J085837.68+182223.4 134.65700 18.37317 0.0589 3.3 11.1 3.46 7.7 40.39 1 -2.88 +2.73 J105842.44+314457.6 164.67683 31.74933 0.0728 4.1 10.0 1.95 6.1 40.71 1 -1.19 +5.33 J105842.58+314459.8 164.67742 31.74994 0.0723 4.1 10.9 2.44 7.5 40.70 1 -1.75 +0.15 J002208.69+002200.5 5.53621 0.36681 0.0710 4.2 11.0 0.02 7.8 40.41 1 -0.02 +0.14 J002208.83+002202.8 5.53679 0.36744 0.0707 4.2 11.2 0.02 8.1 40.44 1 -0.02 +0.49 J133031.75−003611.9 202.63229 −0.60331 0.0542 4.4 8.8 0.35 L 40.44 0 -0.21 +6.69 J133032.00−003613.5 202.63333 −0.60375 0.0542 4.4 10.7 3.98 7.4 40.44 1 -2.57 +0.71 J141447.15−000013.3 213.69646 −0.00369 0.0475 4.9 10.5 0.23 6.9 40.04 1 -0.22 +4.94 J141447.48−000011.3 213.69783 −0.00314 0.0474 4.9 10.2 6.0 40.03 1 3.54 -2.12 J235654.30−101605.4 359.22628 −10.26817 0.0739 4.9 LL L 41.10 1 +0.23 J235654.49−101607.4 359.22708 −10.26875 0.0732 4.9 9.3 2.47 L 41.03 0 -0.17 +0.01 J091931.14+333852.1 139.87977 33.64782 0.0237 5.1 8.5 0.13 L 40.70 0 -0.03 J091930.30+333854.4 139.87628 33.64845 0.0237 5.1 LL L 40.78 0 J093529.56+033923.1 143.87320 3.65644 0.0463 5.4 LL L 41.52 0 +0.11 J093529.77+033918.1 143.87408 3.65505 0.0464 5.4 10.0 0.81 L 41.55 0 -0.13 +0.21 J122814.15+442711.7 187.05896 44.45325 0.0233 5.5 10.7 0.06 6.9 41.06 1 -0.06 J122815.23+442711.3 187.06348 44.45314 0.0229 5.5 LL L 41.01 1 J112648.50+351503.2 171.70212 35.25089 0.0322 5.9 LL L 40.02 1 +0.18 J112648.65+351454.2 171.70274 35.24839 0.0321 5.9 10.0 2.03 6.7 40.01 1 -0.39 +0.13 J090025.61+390349.2 135.10672 39.06369 0.0583 5.9 9.9 0.43 L 40.48 0 -0.07 +9.32 J090025.37+390353.7 135.10572 39.06492 0.0582 5.9 10.1 7.22 8.3 40.48 1 -4.22 +74.29 J151806.13+424445.0 229.52558 42.74585 0.0403 6.2 10.8 50.00 8.4 40.13 1 -31.82 J151806.37+424438.1 229.52664 42.74387 0.0407 6.2 LL L 40.14 1 +4.67 J104518.04+351913.1 161.32520 35.32032 0.0676 6.2 10.6 28.86 7.7 40.30 1 -6.07 +38.08 J104518.43+351913.5 161.32682 35.32041 0.0674 6.2 10.6 27.08 7.0 40.30 1 -16.33 +0.46 J090332.77+011236.3 135.88657 1.21009 0.0580 6.3 10.2 6.7 40.33 0 0.17 -0.15 +0.23 J090332.99+011231.7 135.88747 1.20881 0.0579 6.3 9.7 0.09 L 40.33 0 -0.07 +7.89 J133817.27+481632.3 204.57196 48.27564 0.0278 6.4 10.0 5.48 7.8 40.12 1 -3.33 +3.66 J133817.77+481641.1 204.57404 48.27808 0.0277 6.4 10.6 2.84 8.1 40.13 1 -1.66 +14.39 J114753.63+094552.0 176.97346 9.76444 0.0951 6.6 10.3 8.63 8.6 40.93 1 -5.58 +1.56 J114753.68+094555.6 176.97367 9.76544 0.0966 6.6 11.0 1.10 7.7 40.95 0 -0.63 +0.03 J093634.03+232627.0 144.14185 23.44083 0.0284 6.8 10.8 0.00 7.8 40.51 1 -0.00 +0.39 J093633.93+232638.7 144.14144 23.44411 0.0283 6.8 10.5 1.83 6.9 40.50 0 -0.42 +0.20 J123257.15+091756.1 188.23816 9.29892 0.1048 7.3 11.3 0.03 8.2 41.76 0 -0.03 J123257.38+091757.7 188.23912 9.29939 0.1049 7.3 LL L 41.76 0 +0.34 J135853.78+280346.7 209.72413 28.06300 0.0866 7.4 10.1 0.12 L 41.48 0 -0.11 J135853.66+280342.5 209.72362 28.06182 0.0868 7.4 LL L 41.56 0 J084113.09+322459.6 130.30455 32.41657 0.0684 7.7 LL L 39.92 1 +0.17 J084112.79+322455.1 130.30329 32.41533 0.0696 7.7 10.3 0.13 8.0 39.94 0 -0.07 +2.02 J140737.16+442856.2 211.90487 44.48229 0.1429 7.7 10.8 0.80 7.0 41.14 0 -0.62 J140737.43+442855.1 211.90600 44.48200 0.1430 7.7 LL L 41.14 1 +7.55 J084135.08+010156.1 130.39619 1.03228 0.1106 7.8 10.5 5.88 L 40.74 1 -3.44 J084134.87+010153.9 130.39532 1.03165 0.1105 7.8 LL L 40.74 0 +0.41 J230010.24−000533.9 345.04269 −0.09276 0.1798 7.9 11.5 0.06 8.9 41.33 0 -0.06 +0.69 J230010.17−000531.5 345.04239 −0.09211 0.1797 7.9 11.8 0.09 8.8 41.33 1 -0.09 J112536.15+542257.2 171.40067 54.38264 0.0207 7.9 LL L 39.94 1 +0.03 J112535.23+542314.4 171.39682 54.38741 0.0206 7.9 7.1 0.02 L 39.87 0 -0.01 +0.95 J121247.04+070821.6 183.19604 7.13933 0.1362 8.3 L 0.72 L 42.44 0 -0.39 J121246.84+070823.0 183.19517 7.13975 0.1367 8.3 LL L 42.44 0 +0.09 J102229.47+383538.4 155.62280 38.59401 0.0519 8.6 10.9 0.02 7.6 40.35 0 -0.01 +0.02 J102229.95+383544.7 155.62480 38.59577 0.0523 8.6 9.6 0.02 L 40.35 0 -0.01 +0.18 J004343.80+010216.9 10.93251 1.03805 0.1069 8.8 10.4 0.03 7.7 41.98 0 -0.03 +0.87 J004344.07+010215.1 10.93365 1.03754 0.1070 8.8 10.5 2.95 6.9 41.96 0 -0.43 +5.73 J083817.59+305453.5 129.57329 30.91486 0.0478 8.8 10.7 7.0 40.88 1 2.62 -1.88 +0.28 J083817.95+305501.1 129.57479 30.91697 0.0481 8.8 11.2 0.05 7.3 40.89 0 -0.04 4 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 1 (Continued) Name R.A. Decl. zr log M SFR log M Flag p BH log L * X,lim (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) +0.20 J110713.23+650606.6 166.80511 65.10192 0.0328 8.8 11.2 0.03 8.1 40.62 1 -0.03 J110713.49+650553.2 166.80622 65.09819 0.0319 8.8 LL L 40.53 1 +1.61 J090714.45+520343.4 136.81021 52.06206 0.0596 8.9 10.6 0.59 7.7 40.54 1 -0.52 +2.95 J090714.61+520350.7 136.81087 52.06408 0.0602 8.9 10.3 1.28 7.0 40.55 1 -0.94 +0.18 J145309.42+215404.4 223.28929 21.90123 0.1169 9.2 11.0 0.03 8.3 42.18 0 -0.03 +0.09 J145309.62+215407.8 223.29010 21.90220 0.1155 9.2 10.8 0.01 7.9 42.13 0 -0.01 +0.18 J111828.42−003302.7 169.61845 −0.55077 0.1001 9.3 10.8 0.03 7.4 41.62 0 -0.03 +0.22 J111828.41−003307.8 169.61842 −0.55216 0.1003 9.3 10.3 0.60 6.5 41.62 0 -0.16 +0.41 J134736.41+173404.7 206.90171 17.56797 0.0447 9.3 9.4 L 41.17 1 0.30 -0.18 +0.35 J134737.11+173404.1 206.90462 17.56781 0.0450 9.3 10.5 0.13 6.7 41.18 0 -0.11 J142445.68+333749.4 216.19036 33.63039 0.0710 9.5 LL L 41.48 0 +0.59 J142445.86+333742.7 216.19111 33.62855 0.0718 9.5 10.5 2.47 L 41.50 0 -0.41 +0.97 J000249.07+004504.8 0.70446 0.75133 0.0868 9.5 11.2 0.16 8.6 41.60 1 -0.15 +0.09 J000249.44+004506.7 0.70600 0.75186 0.0865 9.5 10.9 0.01 7.8 41.60 0 -0.01 J094543.54+094901.5 146.43146 9.81709 0.1564 9.6 LL L 41.74 1 +36.44 J094543.78+094901.2 146.43245 9.81700 0.1566 9.6 11.2 25.98 7.5 41.64 0 -14.72 +0.11 J143106.40+253800.0 217.77668 25.63335 0.0964 9.7 10.9 0.02 8.1 41.69 0 -0.02 J143106.79+253801.3 217.77832 25.63370 0.0961 9.7 LL L 41.69 0 +2.10 J085953.33+131055.3 134.97224 13.18205 0.0308 9.7 10.6 0.44 6.9 39.98 1 -0.42 +0.01 J085952.51+131044.3 134.96882 13.17900 0.0297 9.7 10.1 0.01 5.8 39.94 0 -0.01 +0.08 J123515.49+122909.0 188.81454 12.48585 0.0485 9.9 10.3 0.01 6.1 40.06 1 -0.01 +0.03 J123516.05+122915.4 188.81688 12.48763 0.0488 9.9 9.5 0.15 L 40.06 0 -0.03 J161758.52+345439.9 244.49387 34.91109 0.1497 10.0 LL L 41.97 1 J161758.62+345436.3 244.49426 34.91007 0.1492 10.0 LL L 41.96 0 +0.31 J125253.91−031811.0 193.22466 −3.30309 0.0863 10.3 10.6 7.1 41.88 0 0.07 -0.06 J125254.33−031812.1 193.22640 −3.30338 0.0862 10.3 LL L 41.88 0 +0.17 J121514.42+130604.5 183.81009 13.10126 0.1227 10.3 11.1 0.03 8.4 41.71 0 -0.03 +0.33 J121514.17+130601.5 183.80906 13.10043 0.1242 10.3 10.9 0.08 7.4 41.76 0 -0.07 +0.26 J095749.15+050638.3 149.45481 5.11066 0.1217 10.7 11.1 0.04 7.9 41.49 1 -0.04 J095748.95+050642.2 149.45399 5.11174 0.1221 10.7 LL L 41.50 0 +0.04 J123637.31+163351.8 189.15549 16.56441 0.0728 10.7 10.8 0.01 6.9 40.62 0 -0.01 +0.11 J123637.50+163344.6 189.15627 16.56239 0.0733 10.7 11.1 0.02 8.5 40.66 1 -0.02 +0.34 J114608.29−010709.8 176.53458 −1.11940 0.1189 11.2 11.1 0.05 8.2 41.28 0 -0.05 J114608.19−010714.8 176.53414 −1.12078 0.1190 11.2 LL L 41.28 0 J151110.35+054851.7 227.79314 5.81437 0.0799 11.8 LL L 40.43 0 +0.04 J151109.85+054849.3 227.79105 5.81370 0.0803 11.8 10.3 0.01 6.6 40.45 0 -0.01 +21.73 J124545.20+010447.5 191.43836 1.07987 0.1068 11.9 11.3 12.39 8.1 41.35 1 -8.11 +0.16 J124545.13+010453.4 191.43807 1.08153 0.1064 11.9 10.9 0.03 7.1 41.34 0 -0.02 J094130.00+412302.0 145.37504 41.38390 0.0174 12.1 LL L 39.76 0 +0.01 J094132.00+412235.5 145.38339 41.37656 0.0172 12.1 8.6 0.01 L 39.73 0 -0.00 +0.09 J090134.48+180942.9 135.39368 18.16195 0.0665 12.2 10.8 7.3 41.39 1 0.01 -0.01 +0.05 J090135.15+180941.7 135.39646 18.16159 0.0665 12.2 9.7 0.07 L 41.63 0 -0.02 J105622.07+421807.8 164.09208 42.30219 0.0775 12.3 LL L 39.90 1 +0.08 J105622.82+421809.7 164.09518 42.30267 0.0776 12.3 10.3 0.02 L 39.90 0 -0.02 J132924.60+114816.5 202.35253 11.80459 0.0222 12.4 LL L 39.28 0 +0.57 J132924.25+114749.3 202.35108 11.79703 0.0216 12.4 10.5 1.31 6.4 39.25 1 -0.41 J111136.07+574952.4 167.90019 57.83131 0.0472 12.5 LL L 41.11 0 +0.19 J111134.88+574942.8 167.89524 57.82866 0.0465 12.5 9.9 0.46 L 41.27 0 -0.13 +0.12 J135429.06+132757.3 208.62108 13.46592 0.0633 12.5 10.1 0.04 7.0 40.82 1 -0.04 +1.66 J135429.18+132807.4 208.62158 13.46872 0.0634 12.5 10.7 0.64 7.0 40.82 0 -0.52 +0.02 J125725.84+273246.0 194.35769 27.54613 0.0186 12.5 10.2 0.00 7.6 39.23 1 -0.00 +0.00 J125723.56+273259.7 194.34822 27.54993 0.0201 12.5 9.0 0.00 L 39.32 0 -0.00 J011448.67−002946.0 18.70281 −0.49612 0.0338 12.6 LL L 40.18 1 +0.03 J011449.81−002943.6 18.70760 −0.49542 0.0349 12.6 L 0.16 L 40.15 0 -0.03 +4.45 J145051.50+050652.1 222.71458 5.11448 0.0275 12.6 11.0 3.24 7.5 39.92 1 -1.94 J145050.63+050710.8 222.71097 5.11968 0.0282 12.6 LL L 39.94 1 J075311.87+123749.1 118.29946 12.63031 0.0298 12.9 LL L 39.87 0 +0.01 J075313.34+123749.1 118.30561 12.63031 0.0294 12.9 8.3 0.23 L 39.86 0 -0.04 5 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 1 (Continued) Name R.A. Decl. zr log M SFR log M Flag p BH log L * X,lim (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) J134844.49+271044.7 207.18540 27.17911 0.0599 12.9 LL L 40.23 1 +0.08 J134844.48+271055.9 207.18535 27.18220 0.0596 12.9 9.9 0.02 L 40.23 0 -0.02 J141958.98+060320.1 214.99577 6.05560 0.0473 13.4 LL L 40.64 0 +0.02 J141959.06+060305.7 214.99610 6.05161 0.0472 13.4 9.5 0.02 L 40.63 0 -0.01 +0.20 J090005.15+391952.2 135.02159 39.33120 0.0959 14.0 11.3 0.03 8.2 41.62 1 -0.03 J090005.69+391947.4 135.02384 39.32988 0.0968 14.0 LL L 41.64 0 +2.23 J125315.57−031030.2 193.31490 −3.17507 0.0845 14.1 10.4 1.52 6.1 41.71 0 -0.93 +0.43 J125315.99−031036.4 193.31665 −3.17680 0.0852 14.1 10.3 0.74 L 41.66 1 -0.26 +0.07 J135225.64+142919.3 208.10683 14.48869 0.0415 14.1 10.7 0.01 7.4 40.83 0 -0.01 +7.98 J135226.65+142927.5 208.11104 14.49097 0.0406 14.1 11.2 3.74 7.8 40.81 0 -2.65 J141807.91+073232.5 214.53297 7.54237 0.0239 14.2 LL L 40.28 0 +0.02 J141805.96+073226.7 214.52486 7.54077 0.0234 14.2 9.3 0.00 L 40.27 0 -0.00 J125400.79+462752.4 193.50335 46.46458 0.0610 14.5 LL L 41.43 0 +2.06 J125359.62+462750.2 193.49846 46.46397 0.0614 14.5 10.3 5.8 41.37 1 0.68 -0.63 +0.07 J163026.65+243640.2 247.61105 24.61118 0.0623 14.7 10.8 0.01 7.4 40.87 0 -0.01 +0.04 J163026.85+243652.1 247.61189 24.61449 0.0619 14.7 10.0 0.01 L 40.87 0 -0.01 +0.03 J080133.07+141341.6 120.39136 14.22618 0.0538 14.8 9.3 0.22 L 40.00 0 -0.03 +0.08 J080133.94+141334.0 120.38784 14.22821 0.0529 14.8 9.7 0.46 L 39.99 1 -0.11 +0.11 J144804.16+182537.8 222.01737 18.42718 0.0378 15.1 10.6 0.02 7.1 40.00 1 -0.02 +0.01 J144804.23+182558.0 222.01764 18.43277 0.0390 15.1 9.6 0.01 L 40.02 0 -0.00 J151031.75+060007.0 227.63229 6.00195 0.0800 15.2 LL L 41.67 0 +0.05 J151031.66+055957.0 227.63192 5.99919 0.0801 15.2 10.4 0.01 7.4 41.66 0 -0.01 +0.22 J141115.91+573609.0 212.81623 57.60258 0.1062 15.2 11.5 0.03 8.6 41.37 1 -0.03 +0.11 J141115.95+573601.2 212.81638 57.60041 0.1049 15.2 10.7 0.02 8.4 41.36 0 -0.02 J111627.21+570659.1 169.11338 57.11651 0.0469 15.2 LL L 40.98 0 +0.01 J111625.68+570709.8 169.10697 57.11950 0.0464 15.2 9.3 0.24 L 41.00 0 -0.04 +0.50 J115532.11+583532.5 178.88379 58.59246 0.1644 15.4 11.1 0.10 8.1 42.72 0 -0.09 +0.37 J115532.10+583538.0 178.88375 58.59397 0.1634 15.4 11.1 0.06 8.2 42.55 0 -0.06 J142553.53+340452.6 216.47307 34.08129 0.0726 15.4 LL L 40.69 0 +0.07 J142553.20+340442.2 216.47172 34.07840 0.0733 15.4 10.5 0.01 7.1 40.76 0 -0.01 J125917.25−013427.8 194.82191 −1.57440 0.1682 15.5 LL L 42.39 0 +19.47 J125917.14−013422.6 194.82143 −1.57297 0.1679 15.5 10.9 14.68 7.0 42.50 0 -8.08 +0.00 J120429.88+022654.6 181.12451 2.44849 0.0200 15.6 9.1 0.00 L 40.09 0 -0.00 +0.00 J120432.18+022711.1 181.13413 2.45310 0.0200 15.6 9.6 0.00 L 39.98 0 -0.00 +0.03 J125922.72+312213.7 194.84467 31.37050 0.0526 15.8 9.7 0.04 L 40.63 0 -0.02 +0.04 J125922.03+312201.1 194.84180 31.36698 0.0524 15.8 9.9 0.04 L 40.63 0 -0.02 J133525.37+380533.9 203.85570 38.09276 0.0655 16.1 LL L 40.97 1 +0.04 J133525.26+380538.6 203.85305 38.09515 0.0649 16.1 10.0 0.01 5.7 40.96 0 -0.01 +0.49 J142442.81−015929.8 216.17840 −1.99163 0.1746 16.8 11.8 0.07 8.3 42.72 0 -0.07 +8.22 J142442.91−015924.3 216.17881 −1.99011 0.1742 16.8 11.2 7.9 42.88 0 5.82 -3.31 J143541.79+330820.0 218.92417 33.13891 0.1206 16.9 LL L 42.01 1 +0.16 J143542.38+330822.1 218.92666 33.13947 0.1205 16.9 11.2 0.03 7.3 42.01 0 -0.02 +0.03 J085405.94+011111.4 133.52477 1.18650 0.0447 17.0 9.4 0.32 L 40.38 0 -0.06 +0.06 J085405.90+011130.6 133.52459 1.19186 0.0441 17.0 10.2 0.43 6.0 40.37 0 -0.06 +0.17 J102108.45+482855.4 155.28523 48.48206 0.0618 17.1 10.5 0.13 7.8 40.78 0 -0.07 +0.47 J102109.88+482857.2 155.29119 48.48256 0.0615 17.1 10.1 0.18 L 40.78 1 -0.14 +0.04 J111519.23+542310.9 168.83012 54.38636 0.0713 17.1 10.5 0.01 7.6 40.57 0 -0.01 +5.20 J111519.98+542316.7 168.83325 54.38797 0.0704 17.1 11.1 1.72 8.0 40.62 1 -1.60 J112402.95+430901.0 171.01229 43.15028 0.0715 17.3 LL L 40.36 1 +0.63 J112401.84+430857.2 171.00768 43.14922 0.0709 17.3 10.5 1.07 6.9 40.33 1 -0.38 +0.08 J171255.40+640145.3 258.23079 64.02934 0.0811 17.5 10.5 0.01 6.7 40.63 0 -0.01 +0.10 J171255.44+640156.7 258.23090 64.03252 0.0813 17.5 10.1 0.10 L 40.60 0 -0.04 J090215.15+520754.7 135.56311 52.13189 0.1029 17.7 LL L 40.95 0 +0.31 J090215.79+520802.0 135.56578 52.13393 0.1023 17.7 10.2 1.06 L 40.99 1 -0.14 J110418.11+594831.6 166.07545 59.80882 0.1148 17.8 LL L 41.38 0 +0.53 J110419.26+594830.7 166.08019 59.80861 0.1132 17.8 10.3 0.43 5.6 41.42 0 -0.21 +0.07 J143454.22+334934.5 218.72592 33.82625 0.0578 18.0 10.8 0.01 7.3 41.06 0 -0.01 +1.82 J143454.68+334920.0 218.72783 33.82222 0.0587 18.0 10.7 0.64 6.7 41.06 0 -0.58 6 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 1 (Continued) Name R.A. Decl. zr log M SFR log M Flag p BH log L * X,lim (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) J155207.85+273514.6 238.03275 27.58740 0.0747 18.4 LL L 40.20 1 +0.13 J155207.87+273501.6 238.03282 27.58380 0.0748 18.4 11.2 0.02 8.4 40.28 1 -0.02 +0.13 J083902.97+470756.3 129.76239 47.13233 0.0524 18.6 10.7 0.03 7.3 40.62 1 -0.02 +0.30 J083902.50+470814.0 129.76046 47.13722 0.0534 18.6 10.5 1.21 7.6 40.64 0 -0.17 +10.33 J214622.41+000452.1 326.59337 0.08114 0.0754 18.7 10.4 4.72 6.1 41.22 0 -3.39 +6.49 J214623.23+000456.7 326.59679 0.08242 0.0750 18.7 10.9 2.72 7.2 41.22 1 -2.04 J123042.83+103445.3 187.67848 10.57926 0.1636 19.6 LL L 41.88 0 +0.38 J123043.27+103442.9 187.68033 10.57860 0.1636 19.6 11.5 0.06 8.7 41.86 0 -0.06 J161111.72+522645.6 242.79888 52.44607 0.0605 19.7 LL L 40.90 0 +0.81 J161113.52+522649.3 242.80639 52.44709 0.0607 19.7 10.3 0.92 L 40.86 1 -0.41 Note. (1) SDSS names with J2000 coordinates given in the form of “hhmmss.ss+ddmmss.s;” (2)–(3) optical position of the galaxy nucleus; (4) redshift; (5) projected −1 physical separation of galaxies in each pair, in units of kpc; (6) stellar mass, in units of M ; (7) star formation rate, in units of M yr , given by the MPA-JHU DR7 e e catalog; (8) black hole mass estimate inferred from σ assuming the M –σ relation of Gültekin et al. (2009), in units of M ; (9) 0.5–8 keV limiting luminosity for BH e * * −1 source detection, in units of erg s ; (10) flag for X-ray detection, 1 and 0 represent detection and non-detection in X-ray, respectively. (This table is available in machine-readable form.) −6 PSF maps supplied and a false detection probability of 10 . of a given band at the position of each nucleus, following the We then searched for an X-ray counterpart of each optical method of Kashyap et al. (2010). Figure 2 plots the histogram nucleus from the X-ray source lists output by wavdetect, of the 0.5–8 limiting luminosity for both the current sample adopting a matching radius of 2″, an empirically optimal value (listed in Table 1) and the AGN pairs of Hou et al. (2020), given the angular resolution and astrometry accuracy of which have a similar distribution, facilitating a direct comparison between the two sample. Chandra in most cases. This is further justified by a random matching test by artificially shifting the positions of all nuclei by ±10″ in R.A. and decl., which finds on average less than 3.3. NuSTAR Spectral Analysis one coincident match with the detected X-ray sources. We note that no pair in our sample has angular separation less than this To help constrain the presence of intrinsically luminous but matching radius (see the third panel in Figure 1), which means heavily obscured AGNs in the sample galaxies, we utilized two nuclei in a pair would not be matched with one identical archival NuSTAR observations that are sensitive to the hard X-ray counterpart in any case. If the optical nucleus was (10 keV) X-rays from obscured AGNs. Eight pairs in the matched with an X-ray counterpart in any of the three energy current sample have been observed by NuSTAR, with an bands, we consider it to be X-ray detected. effective exposure ranging from 19.5 to 211.3 ks. We note that Source photometry was then calculated using the CIAO tool half of these eight observations were taken as a targeted aprate, which properly handles the counting statistics in the observation to probe the hard X-ray emission from a low-count regime. Source count at a given band was extracted putative AGN. from within the 90% enclosed count radius (ECR). The local The NuSTAR data were downloaded and reprocessed background was evaluated from a concentric annulus with following the standard nupipeline in the software package inner-to-outer radii 2–5 times the 90% ECR for the inner NuSTARDAS v2.1.2. The spectra of each galaxy pair were radius, excluding pixels falling within the 90% ECR of extracted for both focal plane modules A and B (FPMA and neighboring sources, if any. In a few cases where the two FPMB) with nuproducts. A circular region was used to extract nuclei have overlapping 90% ECR, we adopt the 50% ECR for the source spectrum, which has a radius of 60″, approximately photometry. The net photon flux was derived by dividing the equaling 75% ECR. Since the two nuclei in a given pair are not exposure map and corrected for the ECF. well resolved by NuSTAR, the source center was set to be the For the optical nuclei without an X-ray counterpart found by brighter nucleus as seen by Chandra, which is generally wavdetect, we extracted the source and background counts in a consistent with the peak of the NuSTAR-detected signal. The similar way and estimated a 3σ upper limit of the net photon background spectra were extracted from a concentric annulus flux using aprate. If the 3σ lower limit were greater than zero, with an inner radius of 90″ and an outer radius of 150″. It turns the nucleus is regarded as an X-ray detection. Using this more out that three of the eight pairs show no significant signal above quantitative criterion, we recover a few more nuclei with the background, thus they were neglected in the spectral significant X-ray emission that have been filtered by wavdetect. analysis. For the remaining nuclei, we again used aprate to derive a 3σ For the five pairs with significant hard X-ray emission, the upper limit of the net photon flux. spectra were grouped to achieve a signal-to-noise ratio (S/N) The net photon fluxes (or upper limits) were then converted greater than 3 per bin over the energy range of 3–79 keV. We to an unabsorbed luminosity in the 0.5–2 and 2–10 keV bands, follow the method of Zappacosta et al. (2018) to simulate by multiplying a unique conversion factor for a given energy background spectrum using the software NUSKYBGD (Wik band according to the fiducial incident spectrum described in et al. 2014) to account for the spatially dependent background Section 3.1. The net counts, photon fluxes, and luminosities are of NuSTAR. This task aims to compute the relative strengths of listed in Table 2. We have also determined the detection limit different background components and hence well reproduce the 7 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 2 X-Ray Properties of Close Galaxy Pairs Name XR.A. XDecl. Counts F F log L log L HR 0.5−2 2-8 0.5−2 2−10 (1)(2)(3)(4)(5)(6)(7)(8)(9) +8.3 +0.42 +0.26 +0.07 +0.13 +0.02 J102700.56+174900.3 156.75230 17.81692 55.1 2.30 0.79 40.73 40.85 -0.76 -8.2 -0.38 -0.22 -0.08 -0.14 -0.24 +4.9 +0.67 +0.10 +0.00 J085837.53+182221.6 134.65689 18.37266 18.7 2.55 <0.19 40.66 <40.13 -0.95 -4.3 -0.58 -0.11 -0.05 +4.4 +0.57 +0.25 +0.12 +0.42 +0.04 J085837.68+182223.4 134.65689 18.37266 14.2 1.79 0.15 40.51 40.04 -0.83 -3.7 -0.48 -0.13 -0.14 -0.75 -0.17 +2.3 +0.35 +0.34 +0.26 J105842.44+314457.6 164.67683 31.74933 2.6 0.30 <0.46 39.92 <40.70 -0.26 -1.6 -0.23 -0.65 -0.74 +0.16 +10.4 +0.60 +2.00 +0.04 +0.08 J105842.58+314459.8 164.67744 31.74995 96.1 1.36 18.40 40.57 42.30 0.82 -10.3 -0.48 -2.10 -0.19 -0.05 -0.05 +6.0 +0.29 +0.18 +0.01 J002208.69+002200.5 5.53621 0.36681 11.9 0.58 <0.13 40.19 <40.12 -0.87 -4.8 -0.23 -0.22 -0.13 +5.2 +0.22 +0.19 +0.26 +0.37 +0.17 J002208.83+002202.8 5.53679 0.36744 7.8 0.28 0.14 39.87 40.17 -0.41 -4.0 -0.16 -0.13 -0.37 -0.88 -0.59 +4.8 +0.75 +0.57 +0.12 +0.18 +0.16 J133032.00−003613.5 202.63333 −0.60379 17.3 2.31 1.12 40.54 40.82 -0.42 -4.1 -0.63 -0.45 -0.14 -0.23 -0.25 +36.4 +1.50 +1.50 +0.02 +0.02 +0.03 J141447.15−000013.3 213.69646 −0.00376 1188.2 27.60 40.40 41.50 42.26 0.04 -36.0 -1.50 -1.50 -0.02 -0.02 -0.04 +25.0 +1.40 +0.80 +0.03 +0.03 +0.04 J141447.48−000011.3 213.69783 −0.00321 556.3 21.70 11.90 41.40 41.73 -0.42 -24.7 -1.30 -0.90 -0.03 -0.03 -0.04 +24.2 +3.40 +9.16 +0.06 +0.02 +0.03 J235654.30−101605.4 359.22646 −10.26818 526.7 24.30 188.03 41.85 43.33 0.74 -24.0 -3.40 -9.06 -0.07 -0.02 -0.03 +6.3 +21.20 +36.53 +0.34 +0.17 +0.43 J122814.15+442711.7 187.05896 44.45325 14.3 17.80 73.90 40.68 41.89 0.57 -5.2 -13.85 -28.40 -0.65 -0.21 -0.11 +6.0 +34.93 +0.17 +0.18 J122815.23+442711.3 187.06348 44.45314 12.1 71.60 <25.80 41.27 <41.42 -0.42 -4.8 -27.40 -0.21 -0.58 +76.6 +9.51 +10.47 +0.01 +0.01 +0.01 J112648.50+351503.2 171.70213 35.25081 5273.1 469.87 500.68 42.39 43.01 -0.13 -75.9 -9.41 -10.37 -0.01 -0.01 -0.01 +3.9 +0.64 +0.15 +0.03 J112648.65+351454.2 171.70263 35.24838 9.5 1.54 <0.43 39.90 <39.94 -0.83 -3.2 -0.53 -0.18 -0.17 +2.9 +0.30 +0.59 +0.26 +0.18 +0.41 J090025.37+390353.7 135.10567 39.06513 7.8 0.35 1.16 39.79 40.90 0.37 -2.8 -0.21 -0.47 -0.40 -0.22 -0.21 +9.4 +1.30 +0.83 +0.05 +0.13 +0.07 J151806.13+424445.0 229.52559 42.74580 76.4 10.20 2.44 40.92 40.90 -0.68 -9.3 -1.35 -0.69 -0.06 -0.14 -0.10 +4.5 +0.70 +0.11 +0.00 J151806.37+424438.1 229.52648 42.74393 15.1 2.36 <0.26 40.30 <39.93 -0.93 -3.9 -0.61 -0.13 -0.07 +6.1 +0.48 +0.25 +0.10 +0.16 +0.11 J104518.04+351913.1 161.32538 35.32022 28.2 1.84 0.58 40.64 40.74 -0.60 -5.5 -0.42 -0.20 -0.11 -0.19 -0.16 +4.2 +0.31 +0.24 +0.19 +0.16 +0.25 J104518.43+351913.5 161.32676 35.32023 10.6 0.55 0.52 40.11 40.69 -0.11 -3.5 -0.24 -0.19 -0.26 -0.20 -0.38 +10.6 +2.10 +1.47 +0.05 +0.11 +0.07 J133817.27+481632.3 204.57207 48.27566 92.6 16.30 4.94 40.80 40.87 -0.61 -10.5 -2.00 -1.25 -0.06 -0.13 -0.11 +15.5 +2.60 +3.10 +0.04 +0.05 +0.06 J133817.77+481641.1 204.57415 48.27808 209.4 26.00 27.40 41.00 41.61 -0.17 -15.3 -2.50 -3.10 -0.04 -0.05 -0.08 +60.7 +4.55 +7.45 +0.02 +0.01 +0.02 J114753.63+094552.0 176.97337 9.76444 3302.0 112.86 361.19 42.74 43.84 0.44 -60.1 -4.51 -7.38 -0.02 -0.01 -0.02 +3.3 +2.39 +1.26 +0.16 +0.41 +0.05 J093634.03+232627.0 144.14171 23.44080 7.6 5.22 0.79 40.32 40.10 -0.75 -2.7 -1.93 -0.67 -0.20 -0.81 -0.25 +14.6 +0.36 +0.28 +0.04 +0.06 +0.06 J084113.09+322459.6 130.30458 32.41649 187.9 4.00 1.84 40.99 41.25 -0.48 -14.5 -0.35 -0.27 -0.04 -0.07 -0.07 +34.5 +2.40 +3.00 +0.03 +0.02 +0.03 J140737.43+442855.1 211.90597 44.48199 1064.1 38.30 79.30 42.65 43.56 0.21 -34.1 -2.30 -3.00 -0.03 -0.02 -0.03 +20.3 +1.00 +1.10 +0.03 +0.03 +0.06 J084135.08+010156.1 130.39612 1.03229 366.7 12.60 14.30 41.93 42.58 -0.08 -20.1 -1.00 -1.00 -0.04 -0.03 -0.05 +4.7 +0.43 +0.18 +0.14 +0.28 +0.08 J230010.17−000531.5 345.04272 −0.09205 14.7 1.10 0.19 41.32 41.16 -0.73 -4.0 -0.35 -0.12 -0.17 -0.40 -0.21 +65.3 +17.37 +17.06 +0.01 +0.01 +0.02 J112536.15+542257.2 171.40069 54.38269 3830.9 785.97 635.29 42.22 42.72 -0.19 -64.7 -17.19 -16.89 -0.01 -0.01 -0.02 +2.9 +2.17 +1.05 +0.15 +0.37 +0.06 J083817.59+305453.5 129.57323 30.91485 0.77 40.79 -0.73 7.8 5.26 40.55 -2.8 -2.04 -0.60 -0.21 -0.66 -0.26 +2.6 +1.59 +1.38 +0.29 +0.28 +0.33 J110713.23+650606.6 166.80544 65.10198 4.3 1.69 1.50 39.96 40.50 -0.15 -2.0 -1.05 -0.90 -0.42 -0.40 -0.58 +2.3 +1.84 +1.05 +0.23 +0.37 +0.11 J110713.49+650553.2 166.80633 65.09846 3.3 2.61 0.77 40.13 40.19 -0.53 -1.7 -1.32 -0.60 -0.31 -0.66 -0.47 +7.1 +0.54 +1.39 +0.19 +0.08 +0.15 J090714.45+520343.4 136.81026 52.06206 40.7 1.01 7.36 40.27 41.73 0.69 -6.4 -0.40 -1.24 -0.22 -0.08 -0.09 +11.6 +1.05 +2.10 +0.08 +0.04 +0.09 J090714.61+520350.7 136.81087 52.06413 120.9 4.93 19.60 40.97 42.16 0.52 -11.5 -0.97 -2.10 -0.10 -0.05 -0.07 +8.7 +11.50 +4.56 +0.05 +0.17 +0.04 J134736.41+173404.7 206.90178 17.56801 67.8 86.40 9.84 41.95 41.59 -0.82 -8.6 -11.50 -4.12 -0.06 -0.24 -0.09 +5.2 +2.76 +3.09 +0.18 +0.15 +0.28 J000249.07+004504.8 0.70433 0.75128 18.1 5.54 7.21 41.35 42.06 -0.08 -6.3 -2.61 -3.11 -0.28 -0.25 -0.34 +6.4 +0.78 +0.71 +0.31 +0.19 +0.49 J094543.54+094901.5 146.43146 9.81709 13.5 0.74 1.27 41.02 41.84 0.10 -5.2 -0.50 -0.56 -0.49 -0.25 -0.43 +23.1 +0.55 +4.40 +0.21 +0.02 +0.02 J085953.33+131055.3 134.97212 13.18192 477.5 0.88 89.20 39.63 42.22 0.97 -22.8 -0.41 -4.30 -0.27 -0.02 -0.01 +9.7 +0.33 +0.30 +0.15 +0.17 +0.24 J123515.49+122909.0 188.81481 12.48569 31.4 0.82 0.64 40.00 40.48 -0.25 -8.4 -0.28 -0.24 -0.18 -0.21 -0.27 +3.5 +1.43 +0.31 +0.04 J161758.52+345439.9 244.49387 34.91109 3.0 1.39 <0.95 41.25 <41.68 -0.64 -2.3 -0.92 -0.47 -0.36 +5.2 +0.71 +0.68 +0.41 +0.21 +0.79 J095749.15+050638.3 149.45481 5.11066 9.4 0.45 1.08 40.57 41.54 0.21 -0.38 -0.84 -0.27 -0.26 -4.0 -0.50 +13.8 +1.30 +0.37 +0.04 +0.14 +0.02 J123637.50+163344.6 189.15634 16.56247 163.2 15.00 0.96 41.63 41.03 -0.90 -13.7 -1.30 -0.30 -0.04 -0.16 -0.04 +3.1 +0.98 +0.16 +0.01 J124545.20+010447.5 191.43838 1.08009 6.6 2.13 <0.59 41.12 <41.16 -0.85 -2.5 -0.79 -0.20 -0.15 +5.9 +2.90 +0.09 +0.08 J090134.48+180942.9 135.39369 18.16188 27.3 <1.07 12.60 <40.39 42.06 0.92 -5.2 -2.50 -0.10 -0.01 +7.4 +0.11 +0.05 +0.08 +0.15 +0.10 J105622.07+421807.8 164.09197 42.30219 41.1 0.56 0.14 40.25 40.23 -0.68 -6.7 -0.11 -0.05 -0.09 -0.18 -0.12 +4.8 +0.28 +0.17 +0.12 +0.29 +0.10 J132924.25+114749.3 202.35106 11.79699 16.7 0.90 0.18 39.32 39.22 -0.70 -4.2 -0.24 -0.11 -0.13 -0.42 -0.23 +16.2 +1.13 +5.30 +0.15 +0.03 +0.04 J135429.06+132757.3 208.62108 13.46604 234.2 2.80 75.10 40.77 42.79 0.91 -16.0 -0.92 -5.20 -0.17 -0.03 -0.02 +7.1 +0.25 +0.16 +0.17 +0.17 +0.26 J125725.84+273246.0 194.35769 27.54613 21.4 0.52 0.33 38.95 39.35 -0.33 -6.4 -0.22 -0.14 -0.23 -0.24 -0.34 +35.0 +6.64 +4.50 +0.02 +0.02 +0.03 J011448.67−002946.0 18.70286 −0.49634 1097.0 163.66 86.20 41.98 42.29 -0.40 -34.6 -6.57 -4.40 -0.02 -0.02 -0.03 +15.3 +3.10 +1.33 +0.03 +0.06 +0.05 J145051.50+050652.1 222.71453 5.11454 208.4 38.20 9.17 41.16 41.13 -0.68 -15.1 -3.10 -1.53 -0.04 -0.08 -0.05 +1.08 +6.4 +0.96 +0.11 +0.11 +0.18 J145050.63+050710.8 222.71082 5.11957 32.9 3.81 3.41 40.18 40.73 -0.17 -5.8 -0.93 -0.83 -0.12 -0.12 -0.17 +4.6 +0.17 +0.12 +0.19 +0.23 +0.30 J134844.49+271044.7 207.18541 27.17911 10.9 0.31 0.18 39.77 40.12 -0.40 -3.9 -0.13 -0.10 -0.24 -0.34 -0.38 +3.5 +2.00 +0.15 +0.01 J090005.15+391952.2 135.02133 39.33119 8.5 4.97 <0.88 41.39 <41.23 -0.89 -2.9 -1.64 -0.17 -0.11 +2.0 +2.51 +0.26 +-0.01 J125315.99−031036.4 193.31665 −3.17680 2.0 3.00 <0.91 41.07 <41.14 -0.84 -1.3 -1.81 -0.40 -0.16 +3.8 +4.90 +2.52 +0.10 +0.38 +0.04 J125359.62+462750.2 193.49847 46.46392 15.6 19.20 1.83 41.58 41.15 -0.81 -4.3 -5.40 -1.43 -0.14 -0.66 -0.19 +3.3 +0.21 +0.14 +0.17 +0.31 +0.10 J080133.94+141334.0 120.38814 14.22832 7.4 0.44 0.14 39.80 39.89 -0.59 -2.7 -0.17 -0.09 -0.21 -0.47 -0.41 +4.4 +0.47 +0.36 +0.15 +0.24 +0.19 J144804.16+182537.8 222.01737 18.42721 14.0 1.16 0.48 39.93 40.13 -0.50 -3.7 -0.38 -0.26 -0.18 -0.33 -0.29 8 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 2 (Continued) Name XR.A. XDecl. Counts F F log L log L HR 0.5−2 2-8 0.5−2 2−10 (1)(2)(3)(4)(5)(6)(7)(8)(9) +5.4 +1.03 +0.19 +0.03 J141115.91+573609.0 212.81623 57.60258 20.2 1.83 <0.76 41.05 <41.27 -0.77 -4.8 -0.80 -0.25 -0.23 +11.4 +2.16 +1.55 +0.09 +0.12 +0.12 J133525.37+380533.9 203.85554 38.09311 62.8 9.54 4.73 41.33 41.62 -0.42 -11.2 -1.90 -1.31 -0.10 -0.14 -0.17 +4.2 +2.98 +2.09 +0.29 +0.36 +0.22 J143541.79+330820.0 218.92417 33.13891 5.8 3.15 1.61 41.41 41.71 -0.36 -3.0 -1.93 -1.27 -0.41 -0.68 -0.59 +2.9 +0.59 +0.51 +0.24 +0.45 +0.14 J102109.88+482857.2 155.29119 48.48256 5.2 0.81 0.28 40.20 40.33 -0.50 -2.2 -0.41 -0.27 -0.31 -1.58 -0.50 +40.9 +1.70 +6.22 +0.04 +0.01 +0.02 J111519.98+542316.7 168.83312 54.38789 1498.0 17.30 218.16 41.65 43.35 0.82 -40.5 -1.60 -6.16 -0.04 -0.01 -0.02 +7.1 +0.27 +0.13 +0.11 +0.21 +0.16 J112402.95+430901.0 171.01226 43.15025 29.2 0.97 0.22 40.42 40.37 -0.72 -6.5 -0.24 -0.12 -0.12 -0.32 -0.18 +5.2 +0.17 +0.11 +0.17 +0.28 +0.11 J112401.84+430857.2 171.00768 43.14922 12.0 0.35 0.12 39.97 40.09 -0.61 -4.6 -0.14 -0.09 -0.22 -0.60 -0.39 +3.1 +0.23 +0.22 +0.24 +0.28 +0.37 J090215.79+520802.0 135.56578 52.13393 5.8 0.32 0.24 40.26 40.73 -0.30 -2.4 -0.16 -0.15 -0.31 -0.45 -0.49 +4.4 +0.15 +0.09 +0.20 +0.29 +0.10 J155207.85+273514.6 238.03275 27.58741 9.1 0.26 0.10 39.88 40.04 -0.48 -3.7 -0.12 -0.06 -0.29 -0.49 -0.52 +16.3 +0.64 +0.40 +0.03 +0.05 +0.05 J155207.87+273501.6 238.03274 27.58389 234.2 7.68 3.14 41.36 41.56 -0.52 -16.1 -0.62 -0.40 -0.04 -0.06 -0.06 +8.9 +0.86 +2.60 +0.19 +0.05 +0.11 J083902.97+470756.3 129.76228 47.13214 70.6 1.58 20.00 40.35 42.04 0.78 -8.8 -0.67 -2.60 -0.24 -0.06 -0.07 +3.3 +0.94 +0.17 +0.22 J214623.23+000456.7 326.59679 0.08242 6.3 <0.49 1.93 <40.16 41.35 0.78 -2.7 -0.76 -0.22 -0.03 +3.9 +0.75 +0.64 +0.17 +0.22 +0.21 J161113.52+522649.3 242.80594 52.44716 10.4 1.54 0.98 40.47 40.87 -0.32 -3.2 -0.59 -0.48 -0.21 -0.29 -0.39 Note. (1) SDSS names with J2000 coordinates given in the form of “hhmmss.ss+ddmmss.s;” (2)–(3) centroid position of the X-ray counterpart; (4) observed net −6 −2 −1 counts in 0.5–8 (F) keV bands; (5)–(6) observed photon flux in 0.5–2 (S) and 2–8 (H) keV bands, in units of 10 ph cm s ; (7)–(8) 0.5–2 and 2–10 keV −1 unabsorbed luminosities, in units of erg s ; (9) hardness ratio between the 0.5–2 and 2–8 keV bands. (This table is available in machine-readable form.) Gaussian component was not required for the other four sources. The spectral fit results are listed in Table 3, which include the best-fit absorption column density, photon index, 3–79 keV unabsorbed flux and 2–10 keV unabsorbed lumin- osity converted from the best-fit model. 4. Results 4.1. X-Ray Detection Rate A bright X-ray source matched with the galactic nucleus usually refers to an AGN, but with potential contamination from the host galaxy (i.e., nuclear starburst). As estimated in Section 4.2, The X-ray emission due to star formation is neglectable compared to AGN, especially for the nuclei with 41 −1 L > 10 erg s . 2−10 In total, we find 70 X-ray-detected nuclei, among which 67 are detected in the S band, 58 are detected in the H band, and 70 are detected in the F band. Among the 92 close galaxy pairs, 14 pairs have both nuclei detected, 42 pairs have only one of the two nuclei detected, and 36 pairs have no X-ray detection. Figure 3 displays the SDSS gri color-composite images and the Chandra 0.5–8 keV images of the newly-found close galaxy Figure 2. 0.5–8 keV detection limit distribution of the close galaxy pairs pairs with both nuclei detected in the X-rays (the other six pairs studied in this work (black solid histogram), in comparison with the close AGN have been studied and presented in Hou et al. 2020). These 16 pairs (blue dashed) in Hou et al. (2020). The vertical lines mark the median value of the individual samples. nuclei have a 0.5–8 keV luminosity ranging from 40 42 −1 1.3 × 10 –6.3 × 10 erg s . +5% We find an X-ray detection rate of 38% (70/184) among background spectrum at each position of the detector. The -5% the 92 close galaxy pairs. The quoted error takes into account FPMA/FPMB spectra were jointly fitted. Spectral analysis was the counting (Poisson) error in both the numerator and carried out with Xspec v.12.12.1c, adopting the χ statistics to denominator. In the more conservative case, where we only determine the best-fit model. Since we are mainly interested in consider X-ray counterparts with a 2–10 keV unabsorbed constraining the line-of-sight absorption column density and 41 −1 luminosity L > 10 erg s , which are most likely domi- the intrinsic X-ray luminosity, we adopted a phenomenological 2−10 nated by an AGN (see Section 4.2), the detection rate becomes model, an absorbed power law model tbabs∗powerlaw, as the +3% 18% (32/184). For comparison, Hou et al. (2020) gave a default model. In one source, J1451+0507, significant excess is -3% +5% present around 6.4 keV, which can be interpreted as an iron detection rate of 27% (36/134) for their entire sample of -5% fluorescent emission line often seen in luminous AGNs. For AGNs (i.e., regardless of the value of r ) above the threshold of 41 −1 this source, we added a Gaussian component to account for the L = 10 erg s . This factor of ∼1.5 difference may be 2−10 putative Fe line, which significantly improved the fit. Such a understood as a systematically higher fraction of true AGNs in 9 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 3 NuSTAR Spectral Fit Results Name Observation ID N Γ χ /d. o. f F L L H 3−79 2−10,N 2−10,C (1)(2)(3)(4)(5)(6)(7)(8) +7.15 +0.25 +1.78 +0.12 +0.03 J0841+0102 60401002002 0.03 0.61 37.48/48 8.82 43.16 42.58 -0.03 -0.13 -2.05 -0.05 -0.03 +0.13 +2.50 +0.73 +0.06 +0.01 J1125+5423 60160430002 0.78 1.59 199.88/203 8.11 42.34 42.72 -0.78 -0.09 -0.67 -0.04 -0.01 +10.08 +0.41 +1.03 +0.19 +0.05 J1338+4816 60465005002 2.09 1.3 42.13/38 3.17 42.02 41.61 -2.09 -0.25 -0.77 -0.10 -0.05 +4.59 +0.14 +1.32 +0.07 +0.03 J1354+1328 60160565002 17.53 1.45 166.38/183 14.81 43.51 42.79 -3.99 -0.13 -1.19 -0.07 -0.03 +17.78 +0.40 +6.10 +0.18 +0.06 J1450+0507 60301025002 0.02 -0.27 32.88/35 13.08 41.50 41.13 -0.02 -0.32 -4.59 -0.07 -0.08 22 −2 2 Note. (1) Source name; (2) NuSTAR observation ID; (3) best-fit column density, in units of 10 cm ; (4) best-fit photon index; (5) χ over degree of freedom; (6) −12 −1 −2 3–79 keV unabsorbed flux derived from the best-fit spectral model, in units of 10 erg s cm ; (7) 2–10 keV intrinsic luminosity derived from the NuSTAR spectrum; (8) 2–10 keV intrinsic luminosity of the brighter nucleus derived from Chandra data. the Hou et al. (2020) sample, which is consistent with their of which are X-ray detected. The remaining 27 nuclei are original optical classification. When considering the fraction of classified as SF nuclei, but only eight (∼30%) of them are pairs containing at least one X-ray-detected nucleus with X-ray-detected, suggesting that the SF activity does not +7% L > 10 ,we find 32% (30 out of 92 pairs) for the contribute strongly to the observed X-ray emission, at least in 2−10 -7% current sample, which is again somewhat lower than that of the this subset of the sample with optical emission-line measure- +11% ments. We note that only two pairs in our sample have both Hou et al. (2020) AGN sample (47% ; 32 out of 67 pairs). -10% nuclei classified as SF. These and additional detection rates are reported in Table 4. We further use SDSS spectroscopic star-formation rates (SFRs; Brinchmann et al. 2004) provided by the MPA-JHU 4.2. Global X-Ray Properties DR7 catalog to estimate the SF-contributed X-ray luminosity. The left panel of Figure 4 shows L against the hardness 2−10 We note that the SFR is based primarily on the Hα emission ratio of the 70 X-ray-detected nuclei in the close galaxy pairs line, which might be contaminated in the presence of an AGN, (black squares). The hardness ratio, which is defined as but such an effect should lead to an overestimate of the SF- HR = (H − S)/(H + S), is calculated from the observed photon contributed X-ray luminosity, thus strengthening the following flux in the S (0.5–2 keV) and H (2–8 keV) bands using a conclusion. The information of SFR is available for 137 of the Bayesian approach (Park et al. 2006). For the nuclei that are not 184 nuclei in the entire sample. Following Hou et al. (2020), detected in the H band, we show the 3σ upper limit of L by 2−10 we adopt the empirical relation of Ranalli et al. (2003), arrows in the plot. The X-ray counterparts of the AGN pairs (blue circles) and SFG pairs (red triangles) from Hou et al. SFR SF 39 -1 L =´ 4.5 10 erg s , () 1 0.5-2 -1 (2020) are also plotted for comparison (excluding those already M yr included in the new sample). SFR Sixteen nuclei in the current sample are found to have SF 39 -1 L =´ 5.0 10 erg s , () 2 42 −1 21 - 0 -1 L > 10 erg s . These 16 nuclei are probably bona fide M yr 2−10 AGNs, but notably only four of them are found in a pair which has an rms scatter of 0.27 dex and 0.29 dex in the containing another X-ray-detected nucleus (J0907+5203, 0.5–2 keV and 2–10 keV band, respectively. J1058+3144, J1126+3515, and J1414−0000). The majority Figure 5 shows the comparison between the measured X-ray of close galaxy pairs, however, are found at the bottom left 41 −1 luminosity and the empirical X-ray luminosity due to star portion with relatively low luminosities (L < 10 erg s ) 2−10 SF SF formation (L , L ) in the two bands. The majority of the and a negative HR (i.e., a soft spectrum), a region also 0.5-2 21 - 0 detected nuclei lie significantly above the predicted SF- occupied by most SFG pairs. This may suggest that the X-ray contributed luminosity. This holds in both bands, and more emission of these nuclei are dominated by SF activities (e.g., so in the 2–10 keV band. Still, a few SF nuclei (magenta stars) high-mass X-ray binaries and circumnuclear hot gas heated by and a few composite nuclei (orange circles) have their X-ray supernovae) rather than an AGN. However, this does not totally preclude the possibility that some of these sources host luminosity consistent with the predicted SF luminosity. An an accreting SMBH, either intrinsically weak or heavily X-ray AGN is likely absent or heavily obscured in these nuclei. obscured by circumnulcear cold gas with a high column In the meantime, the majority (but all) of the optically classified density. In such a case, the observed soft X-rays probably arise AGN (cyan diamonds) lie significantly above the predicted SF luminosity, indicating that an AGN is indeed powering the further away from the SMBH. observed X-ray emission from these objects. Overall, Figure 5 Seventy-five nuclei in the current sample have reliable 41 −1 suggests that L = 10 erg s can be taken as a practical optical emission-line measurements provided by the MPA-JHU 2−10 threshold above which a genuine AGN is present and SDSS DR7 catalog. For these nuclei, we plot a standard BPT dominates the X-ray emission. Of the 184 nuclei, 32 have diagram (Baldwin et al. 1981), utilizing the line ratios of L above this threshold. Additionally, 83 nuclei have their [O III]/Hβ and [N II]/Hα to provide a canonical diagnosis of 2−10 3σ upper limit above this threshold. We note that only two of their nature, namely, SF, AGN, or SF/AGN composite, as the 92 pairs in the current sample (J0907+5203 and J1414- shown in the right panel of Figure 4. Forty-eight of the 75 0000, shown as green and yellow crosses in Figure 5) have nuclei can be classified as an AGN or composite, the majority both nuclei detected above this threshold. We also test the SF-contributed X-ray luminosity using https://wwwmpa.mpa-garching.mpg.de/SDSS/DR7/ different relations provided in Lehmer et al. (2010), Mineo 10 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Figure 3. SDSS gri-color composite (first and third columns) and Chandra/ACIS 0.5–8 keV (second and fourth columns) images of the eight newly-found close galaxy pairs with both nuclei detected in the X-rays. Each panel has a size of 50″ × 50″ unless otherwise labeled. North is up and east is to the left. Magenta circles denote positions of the optical nuclei. Green circles represent the 90% ECR of local PSF. et al. (2012), and Fragos et al. (2013). The detailed calculation substantially higher. To check the possibility of a buried but and figures are presented in Appendix. The overall distributions intrinsically luminous AGN, we examine the infrared (IR) color are very similar to that derived in Figure 5, which help to of each galaxy pair provided by the Wide-field Infrared Survey confirm AGNs dominate the X-ray emission for nuclei with Explorer (WISE) survey Wright et al. (2010). Specifically, we 41 −1 L = 10 erg s . 2−10 adopt the color of W1 (3.4 μm) − W2 (4.6 μm), which is sensitive to the presence of a luminous AGN (Jarrett et al. 4.3. Obscured AGNs Probed by WISE Color and NuSTAR 2011; Stern et al. 2012; Satyapal et al. 2014). Figure 6 plots Spectra L versus W1 − W2, for both the close galaxy pairs and 2−10 AGN pairs. Given the relatively large WISE PSF The 2–10 keV luminosity, which is derived by assuming a 22 −2 (FWHM ≈ 6″), the two nuclei in many of these pairs are moderate absorption column density N = 10 cm , might be underestimated if the true absorption column density were unresolved and thus share the same value. Nevertheless, this 11 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Table 4 Comparison of X-Ray Detection Rates Sample Sample Size Detection Requirement No. of Detections Detection Rate (1)(2)(3)(4)(5) +3% Nuclei in close pairs 184 log L > 41 32 18% 2−10 -3% −4 +4% Nuclei with M 91 L /L > 10 14 15% BH 2−10 Edd -5% +5% Nuclei in H20 AGN pairs (all) 134 log L > 41 36 27% 2−10 -5% +10% Nuclei in H20 AGN pairs (r  20 kpc) 56 log L > 41 22 39% p 2−10 -10% +7% Close pairs 92 at least one detection and log L > 41 30 32% 2−10 -7% +2% Close pairs 92 dual detection and log L > 41 2 2% 2−10 -2% +5% Close pairs 92 at least one detection and log L > 42 16 17% 2−10 -5% +12% Close pairs (r < 10 kpc) 40 at least one detection and log L > 41 17 41% p 2−10 -13% +8% Close pairs (r > 10 kpc) 52 at least one detection and log L > 41 13 25% p 2−10 -8% +11% H20 AGN pairs (all) 67 at least one detection and log L > 41 32 47% 2−10 -10% +17% H20 AGN pairs (r  20 kpc) 28 at least one detection and log L > 41 19 66% p 2−10 -18% −4 +13% Close pairs both with M 26 at least one detection and L /L > 10 9 33% BH 2−10 Edd -15% Note. Quoted errors, at 1σ, take into account the Poisson error associated with both the umerator and denominator. does not significantly affect our following conclusion, because galaxies. Following Hou et al. (2020), we bin the data points a luminous AGN, when existed, is expected to dominate the (including the upper limits) into several intervals of r and WISE flux. estimate the mean luminosity of each r bin using the Figure 6 shows that most nuclei fall on the blue side of Astronomy SURVial Analysis (ASURV; Feigelson & Nel- W1 − W2 = 0.5, an empirical threshold that separates star- son 1985), a maximum likelihood estimator of the statistical forming galaxies from AGNs (Satyapal et al. 2014). On the properties of censored data, as is the case here. We have chosen 43 −1 other hand, nearly all nuclei with L > 10 erg s have even bins in logarithmic space covering 3.0 kpc „ r „ 20 kpc 2−10 p W1 − W2 > 0.5, finding good agreement between the X-ray and ensured that each bin contain at least 10 nuclei to minimize and IR AGN classifications. A curious exception is the nucleus random fluctuation. We note that the main conclusion below is (J112648.50+351503.2) with the highest L insensitive to the exact choice of bins. The resultant mean 2−10 43 −1 (1.0 × 10 erg s ), which has W1 − W2 ∼ 0.37, but its high 2–10 keV luminosity of the close galaxy sample is shown by X-ray luminosity warrants an AGN classification, This nucleus large black squares. For comparison, the full AGN pair sample is likely accompanied by intense IR starlight of the host galaxy. of Hou et al. (2020) is shown by blue circles, which covers a Also remarkable are a handful of nuclei with W1 − W2 > 0.5 wider range of r up to 100 kpc. The mean 2–10 keV 43 −1 but also with L  10 erg s . Some of these nuclei might luminosities of optically selected single AGNs and SFG pairs, 2−10 host a heavily obscured AGN and have their L significantly taken from Hou et al. (2020) and calculated with ASURV, are 2−10 underestimated. Fortunately, five of these nuclei have an also plotted for comparison (green and red horizontal lines). available NuSTAR spectrum (Section 3.3). In four of the five The two outermost bins (10 kpc  r < 20 kpc) have a mean cases (J0841+0102, J1338+4816, J1354+1238, and J1450 L comparable with each other within the statistical 2−10 +0507), the 2–10 keV luminosity converted from the best-fit uncertainty, which is also comparable to that of optically 41 −1 model to the NuSTAR spectrum is actually two to seven times selected single AGNs (2.6[ ± 0.6] × 10 erg s ). This sug- higher than the default value of L derived from the gests that galaxy interactions have not generally boosted the 2−10 Chandra data (Table 3; marked by magenta stars in Figure 6). AGN activity at such intermediate separations, if the mean In the remaining case (J1125+5423), the NuSTAR spectrum- X-ray luminosity of single AGNs can be taken as the reference based luminosity is actually two times lower, which might level. On the other hand, as noted by Hou et al. (2020) and reflect intrinsic variability. Nevertheless, the absorption column reiterated here, the AGN pairs at similar r show a substantially densities inferred from the NuSTAR spectra are generally higher mean L . This difference might again be understood 2−10 23 −2 moderate, and in all cases lower than 2 × 10 cm (Table 3). as a systematically higher fraction of luminous AGNs in the This suggests that the L in the other nuclei with Hou et al. (2020) sample, which pertains to their optical 2−10 W1 − W2 > 0.5 but without NuSTAR observations are rather classification. We note that a handful of nuclei with the lowest unlikely to have been underestimated by more than a factor L have a value (or upper limit) consistent with the mean of 2−10 40 −1 of 10. SFG pairs (1.3[ ± 0.3] × 10 erg s ), indicating that an AGN is intrinsically weak or absent in these nuclei. At smaller r , the mean L finds its highest value at the p 2−10 4.4. Mean X-Ray Luminosity versus Projected Separation third bin (6.3 kpc < r < 9.0 kpc), which is about an order of The left panel of Figure 7 shows L (or upper limits for magnitude higher than the mean of the two outer bins as well as 2−10 non-detected nuclei) as a function of projected separation r for the mean of single AGNs. The mean L of the second bin is p 2−10 the close galaxy pairs. As mentioned in Section 1, r is taken as also significantly elevated. This might be understood as a sign a proxy for the merger phase, with the smallest r (a few of enhanced SMBH accretion due to merger-driven gas kiloparsecs) indicating the late stage of a merge. A substantial inflows. However, it is noteworthy that the four nuclei with 43 −1 scatter in L over nearly five orders of magnitude exists in the highest luminosities (L  10 erg s ) were targeted 2−10 2−10 this plot, reflecting a wide range of AGN activity in these by Chandra because they were known to be luminous in either 12 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Figure 4. Left: 2–10 keV luminosity vs. hardness ratio. The black squares, blue circles, and red triangles represent X-ray counterparts of close galaxy pairs (current sample), AGN pairs, and SFG pairs (Hou et al. 2020), respectively. Those nuclei undetected in the hard band are marked by arrows. Right: Standard BPT diagram for the nuclei which have reliable optical emission-line measurements (i.e., with a S/N > 3 in each of the four lines). The solid and dashed lines, taken from Kewley et al. (2001) and Kauffmann et al. (2003),define the canonical regions occupied by star-forming nuclei, composite nuclei and AGNs, which are marked by the cyan diamonds, orange circles, and magenta stars, respectively (same in the left panel). Filled and open symbols represent X-ray detected and non-detected nuclei, respectively. Figure 5. 0.5–2 keV (left panel) and 2–10 keV (right panel) luminosity vs. the predicted luminosity due to star-formation activity. The black squares represent X-ray counterparts of the close galaxy pairs. Those nuclei undetected in a given band are marked by arrows. The black solid line indicates a 1:1 relation, with the pair of dashed lines representing the rms scatter (Equations (1) and (2)). The cyan diamonds, magenta stars, and orange circles denote the optically classified AGNs, SF 41 −1 nuclei, and composite nuclei, respectively. The two pairs with both nuclei detected above L = 10 erg s are labeled as green and yellow crosses in the right 2−10 panel. hard X-rays or the IR, which potentially introduces a selection mean value of single AGNs. Overall, L –r relation 2−10 p effect. We find that removing these nuclei from the second and suggests little evidence for merger-induced AGN activity in third r bins results in a mean L much closer to the mean of close galaxy pairs. p 2−10 This is reinforced when the absolute X-ray luminosity is single AGNs. Therefore it remains inclusive whether the upward rising trend between the fourth and third bins is replaced by the X-ray Eddington ratio (L /L ), as shown 2−10 Edd in the right panel of Figure 7. Here L is the Eddington intrinsic. More surprisingly, the mean L continues to Edd 2−10 decrease toward the smallest r . Nine of the 10 nuclei in the luminosity, which scales with an estimated black hole mass (M ) based on the stellar velocity dispersion (σ ) from the innermost bin, in fact, have L (or upper limit) below the BH 2−10 13 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. only four have a 2–10 keV unabsorbed luminosity 43 −1 10 erg s , a conventional threshold for luminous AGNs. Nevertheless, the majority of the nuclei have an X-ray luminosity (or an upper limit in the case of non-detection) significantly above the empirical luminosity due to star- forming activity (Figure 5). This suggests that a weakly accreting SMBH, rather than star formation, is responsible for the observed X-ray emission in most nuclei. Optical line ratios, which are available for 75 nuclei, support this view (Figure 4). We examine whether the X-ray-detection/non-detections are related to the host galaxy properties. By comparing the distributions of redshift, r-band absolute magnitude of the host galaxy, and X-ray detection limit, between the detected and non-detected nuclei, we find that none of these parameters is statistically distinct between the detected and non-detected nuclei. Our visual examination also does not reveal systematic differences in the global morphology (e.g., more disk- dominated) between the detected and undetected subsets. In Figure 8, we further compare the distributions of stellar velocity dispersion (left panel) and SFR (right panel) between the X-ray detected (red histogram) and undetected nuclei (black histogram). The two subsets both show a large scatter in their Figure 6. 2–10 keV luminosity vs. W1 − W2 color. The close galaxy pairs and AGN pairs of Hou et al. (2020) are shown by black squares and blue circles, stellar velocity dispersion, but there is no systematic difference respectively. The solid and open symbols represent detections and non- between the two. This indicates that the detected nuclei are not detections. Error bars are neglected for clarity. The magenta stars mark the preferentially found in galaxies with a more massive SMBH 2–10 keV luminosity derived from NuSTAR spectra, which are available for (assuming that M is statistically reflected by the stellar five pairs. BH velocity dispersion). On the other hand, a larger fraction of high −1 SFR (0.1 M yr ) is found with the detected subset, MPA-JHU catalog and the empirical M –σ relation from BH although, as previously noted, the presence of an AGN may Gültekin et al. (2009). To ensure a reasonable estimate of M , BH cause an overestimate of the SFR. Neglecting this caveat, such we have discarded those nuclei with values lower than 10 M a trend might be taken as evidence that a larger amount of fuel or higher than 10% of the host galaxy mass. In total, 91 nuclei is available in the detected subset for both star formation and have a reliable M and appear in the L /L –r plot. We BH 2−10 Edd p 43 −1 SMBH accretion. note that two of the four nuclei with L > 10 erg s are 2−10 We also examine the relation between stellar mass ratio of thus not included. The mean L /L of the Hou et al. 2−10 Edd the pairs and the observed X-ray luminosity. Only about half (2020) AGN pairs are plotted for comparison, as well as the galaxy pairs (45/92) have reliable stellar mass measurement for mean L /L of the single AGNs derived in a similar way. 2−10 Edd both nuclei. Among them, only 27 galaxy pairs have at least Clearly, the mean L /L of the close galaxy pairs shows 2−10 Edd one X-ray detected nucleus, which is only a small fraction no significant enhancement relative to that of single AGNs at any r bin. compared to the whole sample. There is a tentative trend that the more massive galaxy in a pair is more likely to host a more luminous AGN. 5. Summary and Discussion Since essentially all nuclei have an SF-contributed luminos- 41 −1 ity below L = 10 erg s (Figure 5), it is practical to 2−10 In this work, we have presented the detection and statistical adopt this as the threshold, above which a genuine AGN can be analyses of X-ray nuclei in a newly compiled sample of 92 identified. This allows us to derive the fraction of pairs close galaxy pairs at low redshift (z ¯ ~ 0.07), based on archival containing at least one X-ray-detected nucleus (the case of only Chandra observations. The sample is designed to have one detected nucleus is sometimes referred to as an “offset projected separations 20 kpc and thus representative of the AGN”), which is ∼33% (Section 4.1). Raising the threshold to intermediate-to-late stage of galaxy mergers. Also by design, 42 −1 10 erg s or restricting to dual AGNs (i.e., both nuclei the sample requires no optical emission-line classification of +5% +2% detected) results in a fraction of 17% and 2% , the nuclei, thus it is largely (but not completely) free of -5% -2% respectively. These may serve as a useful point of reference selection bias for or against intrinsic AGN activity. This sample for theoretical and numerical studies of AGN triggering in has similar X-ray detection sensitivity (down to a limiting 40 −1 interacting galaxy pairs, by virtue of our sample being largely luminosity of ∼10 erg s ), redshift, and host galaxy mass unbiased to AGN selection. Applying different definitions of (Figure 1) compared to the close AGN pairs studied by Hou AGNs (e.g., based on a threshold of bolometric luminosity, et al. (2020), but is a factor of about two larger in size, helping X-ray luminosity, or Eddington ratio), existing numerical to relieve concern about a small number statistics. These factors studies, including both idealized galaxy merger simulations together offer an unprecedented opportunity for probing the (e.g., Capelo et al. 2015; Capelo & Dotti 2017; Solanes et al. connection between galaxy interaction and AGN activity through nuclear X-ray emission, which is generally thought 2019) and cosmological simulations (e.g., EAGLE, Rosas- to be a robust diagnostic of AGNs. Guevara et al. 2019; ASTRID, Chen et al. 2023), typically Despite the excellent sensitivity achieved, less than half of predict a dual-AGN fraction of few percent for luminous AGNs the 184 nuclei are firmly detected in the X-rays, among which or accretion rates close to the Eddington limit. This is 14 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. Figure 7. Left: 2–10 keV luminosity as a function of projected separation. The small black squares and blue circles represent close galaxy pairs and the Hou et al. (2020) AGN pairs, respectively. The 3σ upper limit of undetected nuclei are shown by arrows. For each r bin, the mean luminosities of close galaxy pairs and the Hou et al. (2020) AGN pairs are represented by the large black squares and blue circles, respectively. The mean value of single AGNs (star-forming galaxy pairs) from Hou et al. (2020) is shown by the green (red) horizontal solid line, with 1σ error bars represented by the dashed green (red) lines. Right: Similar to the left panel, but for the X-ray Eddington ratio. The same r bins as in the left panel are adopted. Only those nuclei with a reliable black hole mass estimate (Table 1) are included. Figure 8. Stellar velocity dispersion (left panel) and star-formation rate (right panel) distributions of the X-ray detected (red histogram) and undetected nuclei (black histogram). The vertical lines mark the median values. compatible with the above statistics. However, it is noteworthy further confirmed with the current sample of close galaxy pairs, that current simulations still lack the ability of self-consistently although one should bear in mind that the innermost bins are driven determining the accretion rate and the accretion-induced X-ray by a relatively small number of nuclei. Indeed the mean luminosity luminosity, owing primarily to the lack of resolutions down to of the innermost r bin is fully consistent with the mean of optically the sphere of gravitational influence of the SMBH. This is classified single AGNs. The fraction of nuclei with 41 −1 further complicated by the uncertain degree of circumnulear L > 10 erg s ,18%± 3% (Table 4), is even marginally 2−10 obscuration. Hence caution is warranted when comparing the lower than that of the single AGNs (24%± 5%; Hou et al. 2020). observed and predicted AGN fractions. At face value, this suggests that close galaxy interactions do not Hou et al. (2020) revealed a rather surprising trend of decreasing effectively result in boosted AGN activity, which is contradictive mean X-ray luminosity with decreasing projected separation in their with the general prediction of the aforementioned numerical AGN pairs with r  10 kpc. This is reproduced in Figure 7 and simulations, in which tidal torques become stronger at the smaller 15 The Astrophysical Journal, 949:49 (17pp), 2023 June 1 He et al. separations and thus more effective in driving gas to the vicinity of may not occur until shortly after the second pericentric passage, the SMBH. Interestingly, a recent study by Jin et al. (2021) based which lasts for a few tens of megayears (Capelo et al. 2015).At and after this stage, the separation of the two nuclei remain at on SDSS/MaNGA integral-field spectroscopic mapping of low-z no more than 10 kpc, which is consistent with the inner bins in galaxies, also found no significant excess in the AGN fraction in Figure 7.An efficient feedback can explain the moderate any merger phase compared to that of isolated galaxies. The galaxy column densities inferred for at least a subset of the nuclei. The pair sample of Jin et al. (2021) is free of pre-selection of AGN feedback is likely in a kinetic mode mediated by jets and winds characteristics, which is similar to ours. (Yuan & Narayan 2014), given that most nuclei have a low Two physical scenarios were proposed by Hou et al. (2020) Eddington ratio (Figure 7). Future high-resolution radio and to explain the behavior revealed in Figure 7, which we optical spectroscopic observations will be crucial to search for elaborate here. The first is an obscuration effect. In close galaxy direct evidence of this feedback in the close galaxy pairs. pairs, gravitational perturbation can be sufficiently strong to induce gas inflows in one or both galaxies, which in turn result M.H. is supported by the National Natural Science in the accumulation of circumnuclear cold gas that heavily Foundation of China (12203001) and National Postdoctoral obscures even the hard X-rays, regardless of the intrinsic AGN Program for Innovative Talents of China Postdoctoral Science luminosity. Indeed, observational evidence has been gathered Foundation (grant BX2021016). H.L. and Z.L. acknowledge for heavily obscured AGN pairs at kiloparsec separations support by the National Natural Science Foundation of China (Satyapal et al. 2017; Pfeifle et al. 2019; De Rosa et al. 2023). (12225302). S.F. acknowledges support from National Natural However, obscuration cannot be the sole cause of the low-to- Science Foundation of China (No. 12103017) and Natural moderate luminosities observed in most nuclei of our sample, Science Foundation of Hebei Province (No. A2021205001).X. in view of the following countering evidences. On the one L. acknowledges support from NSF grants AST-2108162 and hand, in the five nuclei with a high-quality NuSTAR spectrum, AST-2206499. The authors wish to thank Drs. Yanmei Chen the best-fit foreground absorption column densities (Table 3) and Zongnan Li for helpful discussions. 24 −2 are far below that required (10 cm ) to completely block X-ray photons below a few kilo electron volts. On the other Appendix hand, in a recent attempt of directly detecting circumnuclear Comparison of Star Formation Contribution to X-Ray cold gas in seven pairs of dual-AGNs based on high-resolution Luminosity CO observations, Hou et al. (2023) found no evidence for an 24 −2 Lehmer et al. (2010) calibrated the 2–10 keV X-ray emission equivalent hydrogen column density 10 cm in any of the from both high- and low-mass X-ray binaries (HMXBs and 14 nuclei, which are all included in Hou et al. (2020;10 LMXBs) based on a sample of 17 luminous infrared galaxies included in the current sample). Nevertheless, it remains and presented an empirical correlation between 2 to 10 keV interesting to see whether a dense circumnuclear gas exists in gal luminosity L , SFR, and stellar mass as the several nuclei with the smallest r (5 kpc), which also HX have the lowest apparent L , through higher resolution CO 2−10 gal LM=+abSFR, () A1 HX observations and hard X-ray observations. In the second scenario, most SMBHs in the close galaxy 28 −1 -1 where α = (9.05 ± 0.37) × 10 erg s M and pairs are currently weakly accreting, which is the result of 39 −1 −1 −1 β = (1.62 ± 0.22) × 10 erg s (M yr ) . As estimated negative AGN feedback that have expelled the circumnuclear based on SDSS images, we adopted a uniform factor of 20% gas and prevents the SMBH from maintaining a high level of for the contribution from LMXBs to enclose the stellar mass in accretion. Numerical simulations of idealized galaxy mergers the nuclear region (∼2″). Since only half of the galaxies have suggest that gas inflows may start as soon as the first pericentric stellar mass measurement, the data points are reduced passage of the two galaxies, typically at a physical separation of 10 kpc, while substantial enhancement of SMBH accretion compared to the others (Figure 9, left panel). This relation Figure 9. 2–10 keV luminosity vs. the predicted luminosity due to star-formation activity according to the relation in Lehmer et al. (2010; left panel), Mineo et al. (2012; middle panel), and Fragos et al. (2013; right panel), respectively. The black squares represent X-ray counterparts of the close galaxy pairs. 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Published: Jun 1, 2023

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