Relativistic Mergers of Supermassive Binary Black Holes and their Electromagnetic Signatures

T. Bode, R. Haas, T. Bogdanović, P. Laguna, and D. Shoemaker

Coincident detections of electromagnetic (EM) and gravitational wave (GW) signatures from coalescence events of supermassive black holes are the next observational grand challenge. Such detections will provide the means to study cosmological evolution and accretion processes associated with these gargantuan compact objects. More generally, the observations will enable testing general relativity in the strong, nonlinear regime and will provide independent cosmological measurements to high precision. Understanding the conditions under which coincidences of EM and GW signatures arise during supermassive black hole mergers is therefore of paramount importance. As an essential step towards this goal, we present results from the first fully general relativistic, hydrodynamical study of the late inspiral and merger of equal-mass, spinning supermassive black hole binaries in a gas cloud. We find that variable EM signatures correlated with GWs can arise in merging systems as a consequence of shocks and accretion combined with the effect of relativistic beaming. The most striking EM variability is observed for systems where spins are aligned with the orbital axis and where orbiting black holes form a stable set of density wakes, but all systems exhibit some characteristic signatures that can be utilized in searches for EM counterparts. In the case of the most massive binaries observable by the Laser Interferometer Space Antenna, calculated luminosities imply that they may be identified by EM searches to z ~ 1, while lower mass systems and binaries immersed in low density ambient gas can only be detected in the local universe.

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Acknowledgements

We thank Mike Eracleous for useful comments and suggestions. Bogdanović is grateful to Richard Mushotzky for simulating discussions about low luminosity AGN. This work was supported in part by the NSF grants 0653443, 0855892, 0914553, 0941417, 0903973. Support for Bogdanović provided by NASA through Einstein Postdoctoral Fellowship Award PF9-00061 issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the NASA under contract NAS8-03060. Computations described in this paper were carried out under Teragrid allocation TG-MCA08X009.