Relativistic Mergers of Supermassive Black Holes and their Electromagnetic Signatures

TL;DR

This paper presents the first fully relativistic hydrodynamical simulations of supermassive black hole mergers, revealing electromagnetic signatures correlated with gravitational waves that depend on spin alignment and gas environment, aiding future multi-messenger observations.

Contribution

It introduces the first comprehensive general relativistic hydrodynamical study of supermassive black hole mergers in gas clouds, highlighting potential electromagnetic signatures for multi-messenger detection.

Findings

Variable EM signatures can arise from shocks and accretion during mergers.

Aligned spins and stable density wakes produce distinctive EM variability.

Massive binaries may be detected electromagnetically up to redshift 1.

Abstract

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.