  Recent numerical relativity simulations have shown that the emission of
gravitational waves at the merger of two black holes gives a recoil kick to the
final black hole. We follow the orbits of a recoiling supermassive black hole
(SMBH) in a fixed background potential of a disk galaxy including the effect of
dynamical friction. If the recoil velocity of the SMBH is smaller than the
escape velocity of the galaxy, the SMBH moves around in the potential along a
complex trajectory before it spirals into the galactic center through dynamical
friction. We consider the accretion of gas onto the SMBH from the surrounding
ISM and estimate the X-ray luminosity of the SMBH. We find that it can be
larger than 3x 10^39 erg^-1 or the typical X-ray luminosity of ultra-luminous
X-ray sources, when the SMBH passes the galactic disk. In particular, the
luminosity could exceed ~10^46 erg s^-1, if the SMBH is ejected into the
galactic disk. The average luminosity gradually increases as the SMBH spirals
into the galactic center. We also estimate the probability of finding recoiling
SMBHs with X-ray luminosities of >3x 10^39 erg^-1 in a disk galaxy.
