2021ApJ...915...97X

Supermassive black hole binary systems (SMBHBs) emitting gravitational waves may be traced by periodic light curves. We assembled a catalog of 149 such periodic light curves, and using their masses, distances, and periods, predicted the gravitational-wave strain and detectability of each binary candidate using all-sky detection maps. We found that the International Pulsar Timing Array (IPTA) provides almost uniform sky coverage-a unique ability of the IPTA-and by 2025 will improve NANOGrav's current minimum detectable strain by a factor of 6 and its volume by a factor of 216. Moreover, IPTA will reach detection sensitivities for three candidates by 2025, and 13 by the end of the decade, enabling us to constrain the underlying empirical relations used to estimate supermassive black hole masses. We find that we can in fact already constrain the mass of a binary in Mrk 504 to M < 3.3 x 10⁹ M_☉. We also identify 24 high-mass, high-redshift galaxies that, according to our models, should not be able to host SMBHBs. Importantly, the GW detection of even one of these candidates would be an essentially eternal multimessenger system, and identifying common false-positive signals from nondetections will be useful to filter the data from future large-scale surveys such as LSST.