A kilonova is a transient astronomical event that occurs when two neutron stars or a neutron star and a black hole collide. It is thought to be the optical counterpart of a gravitational wave event, such as those detected by LIGO. Kilonovae are extremely luminous, with peak absolute magnitudes reaching -9 or brighter. They are also very brief, typically lasting only a few days.
The name “kilonova” was first proposed by Tsvi Piran and Dan Maoz in 1992; it is a portmanteau of the SI prefix “kilo-” (1000) and nova (Latin for “new”), because of their high luminosity. The term was later popularized by Brian Metzger, Peter Kartvelishvili and Stephen Smartt.
Kilonovae are thought to be powered by the radioactive decay of heavy elements produced during the collision, such as uranium and thorium. These elements rapidly synthesize in the hot debris left behind from the collision. The resulting light curve from such an event would be very similar to that of a classical nova, but much brighter and shorter-lived.
Kilonovae were first predicted to occur as a result of binary neutron star mergers, but they have also been suggested as possible explanations for short gamma-ray bursts (GRBs), superluminous supernovae (SLSNe), and even some unusual Type Ia supernovae. In all cases, kilonovae are thought to be produced by the rapid accretion of material onto an compact object, such as a newly formed black hole or neutron star. This can happen either via an initial direct impact or through subsequent disk instability leading to dynamical infall.
The most well-studied kilonova case is that of a binary neutron star merger, which was first proposed by Ramesh Narayan, Tsvi Piran and Jakobus van Paradijs in 1992 . Such events were thought to be relatively rare (~1 per galaxy per million years), but this estimate has since increased by several orders of magnitude following the discovery of binary pulsars in 1995 . The first observational evidence for a binary neutron star merger came from the detection of gravitational waves from GW170817 , which was followed up across the electromagnetic spectrum . This event showed many characteristics typical of kilonovae: it had an initial blue color due to lanthanide-rich outflows , then faded over time as these elements decayed; it exhibited strong radioactivity ; its light curve was well fit by models including both r-process nucleosynthesis and dynamical ejecta ; finally, it displayed evidence for off-axis viewing angles , suggesting that GRB 170817A may have actually been powered by jetted emission associated with the central engine .