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

With its capacity to observe $sim 10textasciicircum5-6$ faint active galactic nuclei (AGN) out to redshift $zapprox 6$, Roman is poised to reveal a population of $10textasciicircum4-6, rm M_odot$ 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 $10textasciicircum3-5$ 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.