Hyper-luminous Wandering Massive Black Holes Discovered in the XMM-Newton Catalog

Galaxies are believed to assemble through hierarchical merging. The cosmological simulations find that the merging process should leave many wandering massive black holes in galactic halos. Only a few candidates for such objects have been found. We report the discovery of two ultrasoft hyper-luminous off-nuclear X-ray sources from the XMM-Newton catalog. One has a projected offset of ~1 arcsec (5.2 kpc) from the nucleus of an inactive S0 galaxy at a distance of d_L=2.3 Gpc in the Extended Groth Strip. It was serendipitously detected by XMM-Newton and Chandra in 2000-2002, with characteristic temperature of 0.1-0.2 keV and peak X-ray luminosity of 4X10^43 erg/s. It was not detected in our later follow-up observations, implying a long-term flux variation factor of >14. It has a faint optical counterpart candidate of the absolute V-band magnitude of -15.9 AB mag. The other source has a projected offset of 11.6 arcsec (12.4 kpc) from the nucleus of a barred S0 galaxy at d_L=244 Mpc. It has been detected from 2006 to 1016 with systematic decrease in both the X-ray luminosity (from 7X10^42 erg/s to 4X10^41 erg/s) and the characteristic temperature (from 0.28 keV to 0.14 keV). The X-ray outburst was associated with an optical flare, which suggested that the event started before 2005. We discuss various explanations for both sources and find that they are best explained as massive (10^4-10^5 solar mass) black holes embedded in the nucleus of possibly stripped satellite galaxies, with the X-ray outbursts due to tidal disruption of surrounding stars by the black holes.