Abstract
Abstract It is often assumed that gravitational-wave (GW) events resulting from the merger of stellar-mass black holes are unlikely to produce electromagnetic (EM) counterparts. We point out that the progenitor binary has probably shed a mass ≳10 M ⊙ during its prior evolution. If even a tiny fraction of this gas is retained in a circumbinary disk, the sudden mass loss and recoil of the merged black hole shocks and heats it within hours of the GW event. Whether the resulting EM signal is detectable is uncertain. The optical depth through the disk is likely to be high enough that the prompt emission consists only of photons from its optically thin skin, while the majority may take years to emerge. However, if some mechanism can release more photons in a time comparable to the few-hour energy production time, the peak luminosity of the EM signal could be detectable. For a disk retaining only ∼10−3 of the mass shed in the earlier binary evolution, medium-energy X-rays to infrared emission would be observable hours after the GW event for source distances of ∼500 Mpc. Events like this may already have been observed, but ascribed to unidentified active galactic nuclei. Improved sky localization should eventually allow identification based on spatial coincidence. A detection would provide unique constraints on formation scenarios and potentially offer tests of strong-field general relativity. Accordingly, we argue that the high scientific payoff of an EM detection fully justifies search campaigns.
Highlights
The direct detection of gravitational waves (GWs) from binary black hole mergers GW150914, GW151224, and a possible third event LVT151012 has drawn wide attention (Abbott et al 2016a, 2016b, 2016d)
For binary black hole mergers the common consensus has been that no significant EM counterpart is expected, except for “those in highly improbable environments pervaded by large ambient magnetic fields or baryon densities” as Abbott et al (2016c) state
Observing the signal—If the EM signal appears a few hours after the GW merger, a fairly accurate position is needed for suitable instruments to scan the error box on a similar timescale
Summary
The direct detection of gravitational waves (GWs) from binary black hole mergers GW150914, GW151224, and a possible third event LVT151012 has drawn wide attention (Abbott et al 2016a, 2016b, 2016d). The lack of a corresponding detection by INTEGRAL/SPI-ACS (Savchenko et al 2016), careful reanalysis of the data (Xiong 2016), and reassessment of the low count statistics (Greiner et al 2016) all lead to the conclusion that the Fermi trigger is consistent with a background fluctuation and unlikely to be of astrophysical origin This is perhaps not surprising: the ultra-prompt nature of the Fermi signal implies an extremely small EM source and probably the near-simultaneous formation of the second black hole, ruling out the usual formation channels. We discuss the evolution of the disk under viscous and tidal stresses If any such disk survives until coalescence, it is perturbed both by the GW recoil and by the sudden drop in mass as energy and momentum are radiated away by GWs. In Section 3, we use simple analytic scaling arguments, calibrated against earlier detailed simulations for supermassive black hole mergers, to provide crude estimates of the possible luminosity, delay time, duration, and approximate spectral energy distribution of the resulting signal. The latter would allow new direct tests of strong-field general relativity
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