Multi-messenger Time-domain Signatures Of Supermassive Black Hole Binaries

TL;DR

This paper explores the combined electromagnetic and gravitational wave signals from supermassive black hole binaries, emphasizing the potential for joint multi-messenger observations to advance understanding of these cosmic systems.

Contribution

It establishes the theoretical connection between electromagnetic variability and gravitational wave signals in SMBHBs, identifying promising signatures for future multi-messenger detection.

Findings

Relativistic Doppler boost-induced variability is the most promising electromagnetic signature.

Joint detection feasibility depends on binary mass, period, and PTA sensitivity.

Upcoming PTA data will expand the parameter space for potential multi-messenger observations.

Abstract

Supermassive black hole binaries (SMBHBs) are a natural outcome of galaxy mergers and should form frequently in galactic nuclei. Sub-parsec binaries can be identified from their bright electromagnetic emission, e.g., Active Galactic Nuclei (AGN) with Doppler shifted broad emission lines or AGN with periodic variability, as well as from the emission of strong gravitational radiation. The most massive binaries (with total mass >10^8 M_sol) emit in the nanohertz band and are targeted by Pulsar Timing Arrays (PTAs). Here we examine the synergy between electromagnetic and gravitational wave signatures of SMBHBs. We connect both signals to the orbital dynamics of the binary and examine the common link between them, laying the foundation for joint multi-messenger observations. We find that periodic variability arising from relativistic Doppler boost is the most promising electromagnetic signature to connect with GWs. We delineate the parameter space (binary total mass/chirp mass versus binary period/GW frequency) for which joint observations are feasible. Currently multi-messenger detections are possible only for the most massive and nearby galaxies, limited by the sensitivity of PTAs. However, we demonstrate that as PTAs collect more data in the upcoming years, the overlapping parameter space is expected to expand significantly.