TIME-DEPENDENT MODELS FOR THE AFTERGLOWS OF MASSIVE BLACK HOLE MERGERS

The Laser Interferometer Space Antenna (LISA) will detect gravitational wave signals from coalescing pairs of massive black holes (MBHs) in the total mass range (105-107)/(1 + z) Msun out to cosmological distances. Identifying and monitoring the electromagnetic (EM) counterparts of these events would enable cosmological studies and offer new probes of gas physics around well-characterized MBHs. Milosavljevic and Phinney proposed that a circumbinary disk around a binary of mass ∼106 Msun will emit an accretion-powered X-ray afterglow approximately one decade after the gravitational wave event. We revisit this scenario by using Green's function solutions to calculate the temporal viscous evolution and the corresponding EM signature of the circumbinary disk. Our calculations suggest that an EM counterpart may become observable as a rapidly brightening source soon after the merger, i.e., several years earlier than previously thought. The afterglow can reach super-Eddington luminosities without violating the local Eddington flux limit. It is emitted in the soft X-ray by the innermost circumbinary disk, but it may be partially reprocessed at optical and infrared frequencies. We also find that the spreading disk becomes increasingly geometrically thick close to the central object as it evolves, indicating that the innermost flow could become advective and radiatively inefficient, and generate a powerful outflow. We conclude that the mergers of MBHs detected by LISA offer unique opportunities for monitoring, on humanly tractable timescales, the viscous evolution of accretion flows and the emergence of outflows around MBHs with precisely known masses, spins, and orientations.