  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.
