X-Ray Emission from a Supermassive Black Hole Ejected from the Center of   a Galaxy
  Recent studies have indicated that the emission of gravitational waves at the merger of two black holes gives a kick to the final black hole. If the supermassive black hole at the center of a disk galaxy is kicked but the velocity is not large enough to escape from the host galaxy, it will fall back onto the the disk and accrete the interstellar medium in the disk. We study the X-ray emission from the black holes with masses of ~10^7 M_sun recoiled from the galactic center with velocities of ~600 km s^-1. We find that their luminosities can reach ~>10^39 erg s^-1, when they pass the apastrons in the disk. While the X-ray luminosities are comparable to those of ultra-luminous X-ray sources (ULXs) observed in disk galaxies, ULXs observed so far do not seem to be such supermassive black holes. Statical studies could constrain the probability of merger and recoil of supermassive black holes. 
Yutaka Fujita (Osaka U.)