Kilonova

A kilonova (also called a macronova) is a transient astronomical event that occurs in a compact binary system when two neutron stars (BNS) or a neutron star and a black hole collide. The kilonova, visible over the weeks and months following the merger, is an isotropically expanding luminous afterglow of electromagnetic radiation emitted by the radioactive decay of r-process nuclei synthesized by—and then ejected from—the initial cataclysmic event.

The high sphericity of kilonovae through its early epochs was deduced from the blackbody nature of the spectrum observed for GW170817 / AT2017gfo, the only BNS merger that has been definitively localized as of June 2026.

History

The existence of thermal transient events from neutron star mergers was first introduced by Li & Paczyński in 1998. The radioactive glow arising from the merger ejecta was originally called a mini-supernova, as it is 1⁄10 to 1⁄100 the brightness of a typical supernova, the self-detonation of a massive star. The term kilonova was later introduced by Metzger et al. in 2010 to characterize the peak brightness, which they showed reaches around 1000 times that of a classical nova.

The first candidate kilonova to be found was detected on June 3, 2013, as short gamma-ray burst GRB 130603B by instruments on board the Swift Gamma-Ray Burst Explorer and KONUS/WIND spacecraft, and then imaged by the Hubble Space Telescope 9 and 30 days later.

On October 16, 2017, the LIGO and Virgo collaborations announced the detection of GW170817, the first gravitational wave (GW) shown to have originated from the binary merger of neutron stars. From its kilonova, it would also become the first GW to be definitively pinpointed by its corresponding electromagnetic observation.