A kilonova (also called a ‘macronova’ or an ‘r-process supernova’) occurs when two neutron stars or a neutron star and a black hole merge in a binary system. Strong electromagnetic radiation is emitted due to the decay of heavy r-process ions that are produced and ejected fairly isotropically during the merger process—similar to a faint, short-lived supernova. The inspiral and merging of two compact objects are thought to be a strong source of gravitational waves (GWs). It is also thought to be the progenitor of short gamma-ray bursts and the predominant source of stable r-process elements in the Universe.
The first clear detection of a kilonova was in association with the short-duration gamma-ray burst GRB 130603B. This gamma-ray burst was in a relatively nearby galaxy, enabling the faint infra-red emission from the kilonova to be detected using the Hubble Space Telescope.