A Brief Review of Kilonova (Mergernova) Researchestwo

The neutron star-neutron star and neutron star-black hole mergers may cause the intense radiation of high-frequency gravitational waves (GWs), they can also produce various electromagnetic emissions by ejecting materials outwards. Therefore, these merger events are the most important targets of current multi-messenger astronomical studies. The thermal transient radiation emitted from merger ejecta is called the kilonova or, more generally, the mergernova, which is powered by the radioactive decays of r-process elements, and the energy injection from the central merger product. This phenomenon was first predicted theoretically by Li and Paczynski in 1998, and confirmed by the optical-infrared observations of the GW170817 event in 2017. As a result, the kilonova/mergernova observation had played a crucial role in locating GW170817, identifying its astrophysical origin, and even constraining the nature of the merger product of this event. This paper briefly reviews the primary progress of kilonova/mergernova researches, including the initiative, improvement, and development of the model, the observational discoveries of candidates, and the optical electromagnetic counterpart of GW170817.