The final merger of a pair of massive black holes in a galactic nucleus is
compelled by gravitational radiation. Gravitational waves from the mergers of
black holes of masses (10^5-10^7)(1+z)^{-1} Msun at redshifts of 1-20 will be
readily detectable by the Laser Interferometer Space Antenna (LISA), but an
electromagnetic afterglow would be helpful in pinpointing the source and its
redshift. Long before the merger, the binary "hollows out" any surrounding gas
and shrinks slowly compared to the viscous timescale of a circumbinary disk.
The inner gas disk is truncated at the radius where gravitational torque from
the binary balances the viscous torque, and accretion onto the black holes is
diminished. Initially, the inner truncation radius is able to follow the
shrinking binary inward. But eventually the gravitational radiation timescale
becomes shorter than the viscous timescale in the disk, leading to a merged
black hole surrounded by a hollow disk of gas. We show that the subsequent
viscous evolution of the hollow, radiation-pressure dominated disk will create
a ~10^{43.5}(M/10^6Msun) ergs s^{-1} X-ray source on a timescale
\~7(1+z)(M/10^6Msun)^{1.32} yr. This justifies follow-up monitoring of
gravitational wave events with next-generation X-ray observatories. Analysis of
the detailed light curve of these afterglows will yield new insights into the
subtle physics of accretion onto massive black holes.Comment: 4 pages, 2 figures, Astrophys. J. Lett., in pres
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