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Concept
Kilonova
Summary
A kilonova (also called a macronova) is a transient astronomical event that occurs in a compact binary system when two neutron stars or a neutron star and a black hole merge. These mergers are thought to produce gamma-ray bursts and emit bright electromagnetic radiation, called "kilonovae", due to the radioactive decay of heavy r-process nuclei that are produced and ejected fairly isotropically during the merger process.
The measured high sphericity of the kilonova AT2017gfo at early epochs was deduced from the blackbody nature of its spectrum.
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 mini-supernova, as it is to 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 1000 times that of a classical nova.
The first candidate kilonova to be found was detected as a short gamma-ray burst, GRB 130603B, by instruments on board the Swift Gamma-Ray Burst Explorer and KONUS/WIND spacecraft and then observed using the Hubble Space Telescope 9 and 30 days after burst.
On October 16, 2017, the LIGO and Virgo collaborations announced the first simultaneous detections of gravitational waves (GW170817) and electromagnetic radiation (GRB 170817A and AT 2017gfo) and demonstrated that the source was a binary neutron star merger. This merger was followed by a short GRB (GRB 170817A) and a longer lasting transient visible for weeks in the optical and near-infrared electromagnetic spectrum (AT 2017gfo) located in a relatively nearby galaxy, NGC 4993. Observations of AT 2017gfo confirmed that it was the first secure case of a kilonova. Spectral modelling of AT2017gfo identified the r-process element Strontium which conclusively ties the formation of heavy elements to neutron-star mergers. Further modelling showed the ejected fireball of heavy elements was highly spherical in early epochs.
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