  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.
