Chandra Discovery of a Binary Active Galactic Nucleus in Mrk 739
  We have discovered a binary AGN in the galaxy Mrk 739 using Chandra and Swift
BAT. We find two luminous L_2-10 keV=1.1*10^43 and 1.0*10^42 erg/s, unresolved
nuclei with a projected separation of 3.4 kpc (5.8\pm0.1) coincident with two
bulge components in the optical image. The western X-ray source (Mrk 739W) is
highly variable (2.5x) during the 4-hour 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 AGN. A high level of
star formation combined with a very low L_[O III]/L_2-10 keV ratio cause the
AGN to be missed in optical observations. CO observations of the (3-2) and
(2-1) lines indicate large amounts of molecular gas in the system that could be
driven towards 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\pm0.3) consistent with an efficiently accreting
AGN. Other than NGC 6240, this stands as the nearest case of a binary AGN
discovered to date.
