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 radioactiv
ely 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.
We study high-energy emission from the mergers of neutron star binaries as electromagnetic counterparts to gravitational waves aside from short gamma-ray bursts. The mergers entail significant mass ejection, which interacts with the surrounding mediu
m to produce similar but brighter remnants than supernova remnants in a few years. We show that electrons accelerated in the remnants can produce synchrotron radiation in X-rays detectable at $sim 100$ Mpc by current generation telescopes and inverse Compton emission in gamma rays detectable by the emph{Fermi} Large Area Telescopes and the Cherenkov Telescope Array under favorable conditions. The remnants may have already appeared in high-energy surveys such as the Monitor of All-sky X-ray Image and the emph{Fermi} Large Area Telescope as unidentified sources. We also suggest that the merger remnants could be the origin of ultra-high-energy cosmic rays beyond the knee energy, $sim 10^{15}$ eV, in the cosmic-ray spectrum.
