What is a Kilonova ?

A kilonova is a relatively fast (day/week timescale) and faint visible/infrared (absolute magnitude of -16 at peak luminosity) transient phenomenon. It is radiated by a fairly isotropic ejecta expelled at a fraction of the speed of light. Kilonovae are powered by the radioactive decay of heavy nuclei within the ejecta from the intense bombardment of nuclei lighter than iron by energetic neutrons. This process is known as the r-process, whose physics requires an energetic and extremely neutron-rich environment to be effective. The violent matter ejections resulting from the coalescence of two neutron stars (or a collision between a black hole and a neutron star) can produce an environment conducive to the r-process and has been proposed, two decades ago, as being one of the production sites of the heaviest elements in the Universe.

On 2017, August 17th, a GW signal triggered the two LIGO interferometers; although there was no detection with Virgo, the non-detection helped to strongly constrain the sky localization of GW170817, as due to the antenna pattern of GW detectors, it would have been seen if it had come from other directions. The analysis of the signal revealed that this event originated from the coalescence of two neutron stars located at a distance of about 40 Mpc. About eleven hours after the GW signal, an optical emission at an apparent magnitude of about 17 was detected in the galaxy NGC 4993 and was unambiguously associated with the merger of the two neutron stars. Additional optical and infrared data taken days after the merger event later confirmed that this optical emission was a kilonova powered by r-process channels. The first ever kilonova unambiguously discovered so far!

Gravitational waves and emitted light reveal merger of two neutron stars – and a kilonova
LCO network captures first kilonova
What is a kilonova and how do you detect one? – Dr Phil Evans University of Leicester
“Turning Stars into Gold: The Discovery of the First Kilonova,” Iair Arcavi, Tel Aviv University