Time-Dependent Models for the Afterglows of Massive Black Hole Mergers

TL;DR

This paper models the electromagnetic afterglow of massive black hole mergers, suggesting earlier and brighter X-ray signals than previously thought, which can be observed and monitored to study black hole environments.

Contribution

It introduces a Green's function-based approach to calculate the time-dependent electromagnetic signatures of circumbinary disks post-merger, highlighting earlier observability of afterglows.

Findings

Electromagnetic counterparts may become observable several years earlier than previous estimates.

The afterglow can reach super-Eddington luminosities without violating local limits.

The innermost disk regions may become advective and generate outflows.

Abstract

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.