A Chandra X-Ray Survey of Optically Selected AGN Pairs

Abstract

We present a Chandra archival study of optically selected active galactic nucleus (AGN) pairs at a median redshift . Out of 1286 AGN pairs (with projected separations r p  < 100 kpc and velocity offsets Δ v  < 600 km s −1 ) optically identified from the Sloan Digital Sky Survey Seventh Data Release, we find 67 systems with archival Chandra observations, which represents the largest sample of optically selected AGN pairs studied in the X-ray. Among the 67 AGN pairs, 21 systems have both nuclei detected in the X-ray, 36 have one nucleus detected in the X-ray, and 10 have no X-ray detection. The X-ray detection rate, 78/134 = 58% (±7% 1 σ Poisson errors), is significantly higher than that (23/134 = 17% ± 4%) of a comparison sample of star-forming galaxy pairs, lending support to the optical AGN classification. In the conservative case where X-ray contamination from star formation is removed, the X-ray detection rate becomes 27% ± 4%, consistent with predictions from the latest galaxy merger simulations. The 2–10 keV X-ray luminosity L 2–10 keV increases with decreasing projected separation in AGN pairs for r p ≳ 15 kpc, suggesting an enhancement of black hole accretion even in early-stage mergers. On the other hand, L 2–10 keV appears to decrease with decreasing projected separation at r p ≲15 kpc, which is contradictory to predictions from merger simulations. The apparent decrease in L 2–10 keV of AGN pairs at r p ≲ 15 kpc may be caused by (i) enhanced absorbing columns from merger-induced gas inflows, (ii) feedback effects from early-stage mergers, and/or (iii) small number statistics. Future X-ray studies with larger samples are needed to put our results on firmer statistical ground.