We analyse Chandra , XMM- Newton and HST data of the double-nucleus Ultraluminous Infrared Galaxy ( ULIRG ) , Mrk 463 .
The Chandra detection of two luminous ( \mathrm { L } _ { \mathrm { 2 - 10 keV } } = 1.5 \times 10 ^ { 43 } and 3.8 \times 10 ^ { 42 } erg cm ^ { -2 } s ^ { -1 } ) , unresolved nuclei in Mrk 463 indicates that this galaxy hosts a binary AGN , with a projected separation of \simeq 3.8 kpc ( 3.83 \pm 0.01 arcsec ) .
While the East nucleus was already known to be a Seyfert 2 ( 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 .