The Afterglow of Massive Black Hole Coalescence

The final merger of a pair of massive black holes in a galactic nucleus iscompelled by gravitational radiation. Gravitational waves from the mergers ofblack holes of masses (10^5-10^7)(1+z)^{-1} Msun at redshifts of 1-20 will bereadily detectable by the Laser Interferometer Space Antenna (LISA), but anelectromagnetic afterglow would be helpful in pinpointing the source and itsredshift. Long before the merger, the binary "hollows out" any surrounding gasand shrinks slowly compared to the viscous timescale of a circumbinary disk.The inner gas disk is truncated at the radius where gravitational torque fromthe binary balances the viscous torque, and accretion onto the black holes isdiminished. Initially, the inner truncation radius is able to follow theshrinking binary inward. But eventually the gravitational radiation timescalebecomes shorter than the viscous timescale in the disk, leading to a mergedblack hole surrounded by a hollow disk of gas. We show that the subsequentviscous evolution of the hollow, radiation-pressure dominated disk will createa ~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 ofgravitational wave events with next-generation X-ray observatories. Analysis ofthe detailed light curve of these afterglows will yield new insights into thesubtle physics of accretion onto massive black holes.Comment: 4 pages, 2 figures, Astrophys. J. Lett., in pres