Chandra unveils a binary active galactic nucleus in Mrk 463

We analyse Chandra, XMM-Newton and Hubble Space Telescope (HST) data of the double-nucleus Ultraluminous Infrared Galaxy (ULIRG), Mrk 463. The Chandra detection of two luminous (L2-10 keV = 1.5 x 10(43) and 3.8 x 10(42) erg cm(-2) s(-1)), unresolved nuclei in Mrk 463 indicates that this galaxy hosts a binary active galactic nucleus (AGN), with a projected separation of similar or equal to 3.8 kpc (3.83 +/- 0.01 arcsec). While the East nucleus was already known to be a type 2 Seyfert (and this is further confirmed by our Chandra detection of a neutral iron line), this is the first unambiguous evidence in favour of the AGN nature of the West nucleus. Mrk 463 is therefore the clearest case so far for a binary AGN, after NGC 6240.