A bright off-nuclear X-ray source: A bright off-nuclear X-ray source: a type IIn supernova, a bright ULX or a recoiling super-massive black hole in CXO J122518.6+144545

Abstract: In this Paper we report the discovery of CXO J122518.6+144545; a peculiar X-ray source with a position 3.6+-0.2",off-nuclear from an SDSS DR7 z=0.0447 galaxy. The 3.6" offset corresponds to 3.2 kpc at the distance of the galaxy. The 0.3-8 keV X-ray flux of this source is 5x10^-14 erg cm^-2 s^-1 and its 0.3-8 keV luminosity is 2.2x10^41 erg/s (2.7x10^41 erg/s; 0.5-10 keV) assuming the source belongs to the associated galaxy. We find a candidate optical counterpart in archival HST/ACS g'-band observations of the field containing the galaxy obtained on June 16, 2003. The observed magnitude of g'=26.4+-0.1 corresponds to an absolute magnitude of -10.1. We discuss the possible nature of the X-ray source and its associated candidate optical counterpart and conclude that the source is either a very blue type IIn supernova, a ULX with a very bright optical counterpart or a recoiling super-massive black hole.