Kilonova/Macronova Emission from Compact Binary Mergers

We review current understanding of kilonova/macronova emission from compact binary mergers (mergers of two neutron stars or a neutron star and a black hole). Kilonova/macronova is optical and near-infrared emission powered by radioactive decays of r-process nuclei. Emission from the dynamical ejecta with ~0.01 Msun is likely to have a luminosity of ~10^{40}-10^{41} erg s^{-1} with a characteristic timescale of about 1 week. The spectral peak is located in red optical or near-infrared wavelengths. A subsequent accretion disk wind may provide an additional luminosity, or an earlier/bluer emission if it is not absorbed by the precedent dynamical ejecta. The detection of near-infrared excess in the afterglow of short GRB 130603B and possible optical excess in GRB 060614 supports the concept of the kilonova/macronova scenario. At 200 Mpc distance, a typical brightness of kilonova/macronova with 0.01 Msun ejecta is expected to be about 22 mag and the emission rapidly fades to >24 mag within ~10 days after the merger. Kilonova/macronova candidates can be distinguished from supernovae by (1) the faster time evolution, (2) fainter absolute magnitudes, and (3) redder colors. To effectively search for such objects, follow-up survey observations with multiple visits within <10 days and with multiple filters will be important. Since the high expansion velocity (v ~ 0.1-0.2c) is a robust outcome of compact binary mergers, the detection of smooth spectra will be the smoking gun to conclusively identify the GW source.