  The Laser Interferometer Space Antenna (LISA) will detect gravitational wave
signals from coalescing pairs of massive black holes in the total mass range
(10^5 - 10^7)/Msol out to cosmological distances. Identifying and monitoring
the electromagnetic counterparts of these events would enable cosmological
studies and offer new probes of gas physics around well-characterized massive
black holes. Milosavljevic & Phinney (2005) proposed that a circumbinary disk
around a binary of mass ~10^6 Msol 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 electromagnetic signature of
the circumbinary disk. Our calculations suggest that an electromagnetic
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 massive black holes detected by LISA
offer unique opportunities for monitoring on humanly tractable timescales the
viscous evolution of accretion flows and the emergence of outflows around
massive black holes with precisely known masses, spins and orientations.
