RADIOACTIVELY POWERED EMISSION FROM BLACK HOLE–NEUTRON STAR MERGERS
Detection of the electromagnetic counterparts of gravitational wave (GW) sources is important to unveil the nature of compact binary coalescences. We perform three-dimensional, time-dependent, multi-frequency radiative transfer simulations for radioactively powered emission from the ejecta of black hole (BH)–neutron star (NS) mergers. Depending on the BH to NS mass ratio, spin of the BH, and equations of state of dense matter, BH–NS mergers can eject more material than NS–NS mergers. In such cases, radioactively powered emission from the BH–NS merger ejecta can be more luminous than that from NS–NS mergers. We show that, in spite of the expected larger distances to BH–NS merger events, the observed brightness of BH–NS mergers can be comparable to or even higher than that of NS–NS mergers. We find that, when the tidally disrupted BH–NS merger ejecta are confined to a small solid angle, the emission from BH–NS merger ejecta tends to be bluer than that from NS–NS merger ejecta for a given total luminosity. Thanks to this property, we might be able to distinguish BH–NS merger events from NS–NS merger events by multi-band observations of the radioactively powered emission. In addition to the GW observations, such electromagnetic observations can potentially provide independent information on the progenitors of GW sources and the nature of compact binary coalescences.