An Etymological Dictionary of Astronomy and Astrophysics

An → electromagnetic signal associated with the location on the sky and the time of a → gravitational wave event. The electromagnetic signal is predicted by models to be associated with the → merger of a → compact binary star system composed of two → neutron stars (NS) or a neutron star and a → black hole (BH).

Accordingly, the gravitational waves are accompanied by a short-duration → gamma-ray burst (GRB) powered by the → accretion of material that remains in a → centrifugally supported → torus around the BH following the merger.

NS-NS/BH-NS mergers are also predicted to be accompanied by a more isotropic counterpart, commonly known as a → kilonova. Kilonovae are day to week-long thermal, → supernova-like → transients, and are powered by the → radioactive decay of heavy, neutron-rich elements synthesized by the → r-process in the expanding merger ejecta (Li and Paczynski 1998). The first detection of an electromagnetic counterpart to gravitational waves belongs to → GW170817.

See also: → electromagnetic; → counterpart.