Massive Black Hole Binaries as LISA Precursors in the Roman High Latitude Time Domain Survey

TL;DR

Roman's high-latitude time domain survey will detect thousands of faint AGN, identify hundreds of massive black hole binaries with short orbital periods, and serve as precursors to LISA gravitational wave sources, enabling multi-messenger astrophysics.

Contribution

This study proposes using Roman's survey to identify massive black hole binaries as LISA precursors, linking electromagnetic observations with gravitational wave detections.

Findings

Roman can observe 10^4-10^6 black holes up to z≈6.

Roman can identify hundreds of MBHBs with 5-12 day periods.

These binaries will merge within 10^3-10^5 years, detectable by LISA.

Abstract

With its capacity to observe $\sim 10^{5-6}$ faint active galactic nuclei (AGN) out to redshift $z\approx 6$, Roman is poised to reveal a population of $10^{4-6}\, {\rm M_\odot}$ black holes during an epoch of vigorous galaxy assembly. By measuring the light curves of a subset of these AGN and looking for periodicity, Roman can identify several hundred massive black hole binaries (MBHBs) with 5-12 day orbital periods, which emit copious gravitational radiation and will inevitably merge on timescales of $10^{3-5}$ years. During the last few months of their merger, such binaries are observable with the Laser Interferometer Space Antenna (LISA), a joint ESA/NASA gravitational wave mission set to launch in the mid-2030s. Roman can thus find LISA precursors, provide uniquely robust constraints on the LISA source population, help identify the host galaxies of LISA mergers, and unlock the potential of multi-messenger astrophysics with massive black hole binaries.