Detection of 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, 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 nature of compact binary coalescences.
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