A Chandra X-ray Survey of Optically Selected AGN Pairs
  We present a Chandra archival study of optically selected AGN pairs at a
median redshift $\bar{z} \sim$ 0.1. Out of 1286 AGN pairs (with projected
separations $r_p < 100$ kpc and velocity offsets $\Delta v < 600$ km s$^{-1}$)
optically identified from the Sloan Digital Sky Survey Seventh Data Release, we
find 67 systems with archival Chandra observations, which represent 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-rays, 36 have one nucleus
detected in the X-rays, whereas 10 have no X-ray detection. The X-ray detection
rate, 78/134 = 58% ($\pm 7$% 1$\sigma$ Poisson errors), is significantly higher
than that (23/134 = $17\% \pm 4\%$) 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 $27\% \pm 4\%$, consistent with predictions from
latest galaxy merger simulations. The 2-10 keV X-ray luminosity $L_{2-10~{\rm
keV}}$ increases with decreasing projected separation in AGN pairs for $r_p
\gtrsim 15$ kpc, suggesting an enhancement of black hole accretion even in
early-stage mergers. On the other hand, $L_{2-10~{\rm keV}}$ appears to
decrease with decreasing projected separation at $r_p \lesssim 15$ kpc, which
is contradictory to predictions from merger simulations. The apparent decrease
in $L_{2-10~{\rm keV}}$ of AGN pairs at $r_p \lesssim 15$ 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.
