Enhanced activity in close dual-AGN systems in the local Universe

We present a study of an X-ray-selected sample of active galactic nuclei (AGN) in pairs at projected spatial separations 1 < r(p)/kpc < 100 at z < 0.1, using XMM-Newton and Chandra data. We derived the pair sample from an initial pool of approximately 2000 X-ray-selected AGN, and is composed of both AGN-AGN pairs (so-called dual AGN) and AGN-galaxy pairs. From this selection, we find that approximately 10% of AGN reside in pairs, and about 4% are paired with another AGN. We performed a detailed X-ray and SDSS optical spectral analysis for AGN in dual systems and X-ray analysis for AGN in AGN-galaxy pairs, to characterise their absorption properties and investigate the possible triggering mechanisms. We then investigated how obscuration, luminosity, and Eddington ratio depend on projected separation r(p). Amongst all AGN in pair, we find that similar to 55% are obscured (with hydrogen column density N-H > 10(22) cm(-2)), amongst which similar to 6% are Compton-thick (N-H > 10(24) cm(-2)). The fraction of absorbed AGN is significantly higher in late-stage mergers (r(p) < 30 kpc) compared to early-stage mergers (r(p) > 60 kpc). Amongst the AGN pairs, we also observe an average excess relative to a control sample of inactive galaxies in pairs, and this excess significantly increases with decreasing r(p), but only for obscured AGN. Finally, in dual-AGN systems, both the bolometric luminosity and the Eddington ratio of the less massive black hole in the pair increase as the separation decreases. These findings suggest that mergers may play an important role in triggering AGN accretion and activity.