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 ...
