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

In this paper we report the discovery of CXO J122518.6+144545, a peculiar X-ray source with a position 3.6 +/- 0.2 arcsec off-nuclear from a Sloan Digital Sky Survey Data Release 7 (SDSS DR7) z = 0.0447 galaxy. The 3.6 arcsec offset corresponds to 3.2 kpc at the distance of the galaxy. The 0.3-8 keV X-ray flux of this source is 5 x 10-14 erg cm-2 s-1 and its 0.3-8 keV luminosity is 2.2 x 1041 erg s-1 (2.7 x 1041 erg s-1; 0.5-10 keV) assuming that the source belongs to the associated galaxy. We find a candidate optical counterpart in archival Hubble Space Telescope/Advanced Camera for Surveys g'-band observations of the field containing the galaxy obtained on 2003 June 16. 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, an ultraluminous X-ray source with a very bright optical counterpart or a recoiling supermassive black hole.