Identifying Decaying Supermassive Black Hole Binaries from their Variable Electromagnetic Emission

TL;DR

This paper explores how variable electromagnetic signals from supermassive black hole binaries can aid in their identification and study, complementing gravitational wave detections by LISA, and discusses observational strategies for detecting these EM signatures.

Contribution

It proposes methods to identify electromagnetic counterparts to SMBHBs detected by LISA through variable EM signals and discusses their potential detectability and scientific implications.

Findings

Variable EM flux can indicate SMBHBs prior to coalescence.

Shocks in circumbinary disks produce transient EM signals.

Deep optical surveys can statistically identify SMBHB populations.

Abstract

Supermassive black hole binaries (SMBHBs) with masses in the range 10^4-10^7 M_sun/(1+z), produced in galaxy mergers, are thought to complete their coalescence due to the emission of gravitational waves (GWs). The anticipated detection of the GWs by the LISA will constitute a milestone for fundamental physics and astrophysics. While the GW signatures themselves will provide a treasure trove of information, if the source can be securely identified in electromagnetic (EM) bands, this would open up entirely new scientific opportunities, to probe fundamental physics, astrophysics, and cosmology. We discuss several ideas, involving wide-field telescopes, that may be useful in locating electromagnetic counterparts to SMBHBs detected by LISA. In particular, the binary may produce a variable electromagnetic flux, such as a roughly periodic signal due to the orbital motion prior to coalescence, or a prompt transient signal caused by shocks in the circumbinary disk when the SMBHB recoils and "shakes" the disk. We discuss whether these time-variable EM signatures may be detectable, and how they can help in identifying a unique counterpart within the localization errors provided by LISA. We also discuss a possibility of identifying a population of coalescing SMBHBs statistically, in a deep optical survey for periodically variable sources, before LISA detects the GWs directly. The discovery of such sources would confirm that gas is present in the vicinity and is being perturbed by the SMBHB - serving as a proof of concept for eventually finding actual LISA counterparts.