2011ApJ...735L..42K

We have discovered a binary active galactic nucleus (AGN) in the galaxy Mrk 739 using Chandra and Swift BAT. We find two luminous (L_2-10 keV_= 1.1x10⁴³ and 1.0x10⁴² erg/s), unresolved nuclei with a projected separation of 3.4 kpc (5".8±0".1) coincident with two bulge components in the optical image. The western X-ray source (Mrk 739W) is highly variable (x 2.5) during the 4 hr Chandra observation and has a very hard spectrum consistent with an AGN. While the eastern component was already known to be an AGN based on the presence of broad optical recombination lines, Mrk 739W shows no evidence of being an AGN in optical, UV, and radio observations, suggesting the critical importance of high spatial resolution hard X-ray observations (>2 keV) in finding these binary AGNs. A high level of star formation combined with a very low L_[O III]_/L2-10 keV_ ratio cause the AGN to be missed in optical observations. ¹²CO observations of the (3-2) and (2-1) lines indicate large amounts of molecular gas in the system that could be driven toward the black holes during the violent galaxy collision and be key to fueling the binary AGN. Mrk 739E has a high Eddington ratio of 0.71 and a small black hole (log M_BH= 7.05±0.3) consistent with an efficiently accreting AGN. Other than NGC 6240, this stands as the nearest case of a binary AGN discovered to date.