The Afterglow of Massive Black Hole Coalescence

Abstract

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

Observational probes of cosmic acceleration

“…One thus needs an identification of the host galaxy, and given the angular positioning accuracy of gravitational wave observations this will generally require identification of an electromagnetic transient that accompanies the gravitational wave burst. Possibilities include GRBs resulting from neutron star mergers (Dalal et al, 2006) and the optical, X-ray, or radio signatures of the response of an accretion disk to a massive black hole merger (Milosavljevic and Phinney, 2005;Lippai et al, 2008). However, both the event rates and the characteristics of the electromagnetic signatures are poorly understood at present.…”

Flares in gamma-ray bursts: disc fragmentation and evolution

“…Milosavljevic & Phinney 2005, Tanaka & Menou 2010. We compute the viscous evolution of the clump semianalytically by solving the standard equation for axisymmetric accretion discs,…”

Cosmological physics with black holes (and possibly white dwarfs)

“…Indeed, one can use near real-time GW information on the sky localization, in combination with the accurate timing of the inspiral event, to predetermine well in advance where on the sky the merger is located. A unique host galaxy identification could then proceed through coordinated observations with traditional telescopes, by monitoring in real time the sky area for unusual electromagnetic emission, as A variety of mechanisms exist through which disturbed gas in the vicinity of black hole pairs will power electromagnetic emission during and after coalescence (Armitage & Natarajan, 2002;Milosavljevic & Phinney, 2005;Dotti et al, 2006;Bode & Phinney, 2007;MacFadyen & Milosavljevic, 2008). For example, at the time of coalescence, ∼ > 10 53 ergs of kinetic energy are delivered to the recoiling black hole remnant and its environment, for typical recoil velocities ∼ > 100 km/s (e.g., Baker et al, 2006Baker et al, , 2007Campanelli et al, 2007;Herrmann et al, 2007;Schnittman & Buonanno, 2007).…”

“…From the black hole masses, spins and binary orientation, all accurately constrained by the GW signal, one would be able to study the physics of the post-merger accretion flow onto the remnant black hole (Milosavljevic & Phinney, 2005;Dotti et al, 2006) with unprecedented accuracy. This would include precise constraints on the Eddington ratio of the accreting source, its emission and absorption geometries and possibly its jet phenomenology.…”