Contaminating Electromagnetic Transients in LISA Gravitational-wave Localization Volumes. I. The Intrinsic Rates

The Laser Interferometer Space Antenna (LISA) will soon detect gravitational waves (GWs) emitted by massive black hole (MBH) mergers. Some theoretical models have predicted transient electromagnetic (EM) emission from these mergers, enabling the association of LISA GW sources with their EM counterparts via telescope follow-up. However, the number of unrelated EM transients that might contaminate telescope searches for the true transient counterparts of LISA MBH mergers is unknown. We investigate the expected numbers of unrelated EM transients that will coincide with simulated LISA localization volumes of MBH mergers as a function of the merger total mass and redshift. We find that the number of potential contaminants in LISA localization volumes drops to unity for mergers at z ≲ 0.8 and at 1 hr before coalescence. After coalescence, the parameter space corresponding to a maximum of one potential contaminant expands to z ≲ 1.5. In contrast, if the redshifts for all transients detected in LISA sky localization regions are not available, the number of potential contaminants increases by an average factor of ∼100 and never drops below unity. Overall, we expect the average number of contaminating transients in telescope follow-up of LISA MBH mergers to be nonnegligible, especially without redshift information for the detected transients. We recommend that endeavors designing follow-up strategies of LISA events should focus on (1) building large redshift catalogs for host galaxies, (2) developing robust real-time transient classification algorithms, and (3) coordinating telescope resources to obtain redshifts for candidate transient EM counterparts in a timely manner.