Authors: Zoltan Haiman, Chengcheng Xin, Tamara Bogdanović, Pau Amaro Seoane, Matteo Bonetti, J. Andrew Casey-Clyde, Maria Charisi, Monica Colpi, Jordy Davelaar, Alessandra De Rosa, Daniel J. D'Orazio, Kate Futrowsky, Poshak Gandhi, Alister W. Graham, Jenny E. Greene, Melanie Habouzit, Daryl Haggard, Kelly Holley-Bockelmann, Xin Liu, Alberto Mangiagli, Alessandra Mastrobuono-Battisti, Sean McGee, Chiara M. F. Mingarelli, Rodrigo Nemmen, Antonella Palmese, Delphine Porquet, Alberto Sesana, Aaron Stemo, Alejandro Torres-Orjuela, Jonathan Zrake
With its capacity to observe 105−6 faint active galactic nuclei (AGN) out to redshift z≈6, Roman is poised to reveal a population of 104−6Msolar 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 103−5 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.