The high sphericity of kilonovae through its early epochs was deduced from the blackbody nature of the spectrum observed for the most important recordedBNS merger,GW170817 / AT2017gfo.[4][5]
Animation showing two small, very dense neutron stars merge via gravitational wave radiation and explode as a kilonova
The existence of thermal transient events from neutron star mergers was first introduced by Li &Paczyński in 1998.[1] The radioactive glow arising from the merger ejecta was originally called mini-supernova, as it is1⁄10 to1⁄100 the brightness of a typicalsupernova, the self-detonation of a massive star.[6] The termkilonova was later introduced by Metzger et al. in 2010[7] to characterize the peak brightness, which they showed reaches 1000 times that of a classicalnova.
This artist's impression shows a kilonova produced by two colliding neutron stars.
On October 16, 2017, theLIGO andVirgo collaborations announced the detection ofGW170817,[9] the firstgravitational wave (GW) shown to have originated from the binarymerger ofneutron stars.[10] From its kilonova, it would also become the first GW to be definitively pinpointed by its corresponding electromagnetic observation. The GW detection co-occurred with a short GRB(GRB 170817A), and then after several hours, a longer lasting astronomicaltransient (AT 2017gfo), visible for weeks in the optical and near-infrared electromagnetic spectrum.
The kilonova observations allowed the event to be precisely located at just 140 million light-years away in the nearby galaxyNGC 4993.[11] Observations ofAT 2017gfo confirmed that it was the first conclusive observation of a kilonova.[12] Spectral modelling of AT2017gfo identified ther-process elementsstrontium andyttrium, which conclusively ties the formation of heavy elements to neutron-star mergers.[13][14] Further modelling showed the ejected fireball of heavy elements was highly spherical in early epochs.[4][15] Some researchers have suggested that "thanks to this work, astronomers could use kilonovae as astandard candle to measure cosmic expansion. Since kilonovae explosions are spherical, astronomers could compare the apparent size of a supernova explosion with its actual size as seen by the gas motion, and thus measure the rate of cosmic expansion at different distances."[16]
Theinspiral and merging of twocompact objects are a strong source ofgravitational waves (GW).[7] The basic model for thermal transients from neutron star mergers was introduced byLi-Xin Li andBohdan Paczyński in 1998.[1] In their work, they suggested that the radioactive ejecta from a neutron star merger is a source for powering thermal transient emission, later dubbedkilonova.[17]
A first observational suggestion of a kilonova came in 2008 following thegamma-ray burst GRB 080503,[19] where a faint object appeared in optical light after one day and rapidly faded. However, other factors such as the lack of a galaxy and the detection of X-rays were not in agreement with the hypothesis of a kilonova. Another kilonova was suggested in 2013, in association with theshort-duration gamma-ray burst GRB 130603B, where the faintinfrared emission from the distant kilonova was detected using theHubble Space Telescope.[8]
In October 2017, astronomers reported that observations ofAT 2017gfo showed that it was the first definitive case of a kilonova following amerger of twoneutron stars.[12]
In October 2018, astronomers reported thatGRB 150101B, agamma-ray burst event detected in 2015, may be analogous to the historicGW170817. The similarities between the two events, in terms ofgamma ray,optical andx-ray emissions, as well as to the nature of the associated hostgalaxies, are considered "striking", and this remarkable resemblance suggests the two separate and independent events may both be the result of the merger of neutron stars, and both may be a hitherto-unknown class of kilonova transients. Kilonova events, therefore, may be more diverse and common in the universe than previously understood, according to the researchers.[20][21][22][23] In retrospect, GRB 160821B, a gamma-ray burst detected in August 2016, is now believed to also have been due to a kilonova, by its resemblance of its data toAT2017gfo.[24]
A kilonova was also thought to have caused thelong gamma-ray burstGRB 211211A, discovered in December 2021 bySwift’s Burst Alert Telescope (BAT) and theFermi Gamma-ray Burst Monitor (GBM).[25][26] These discoveries challenge the formerly prevailing theory that long GRBs exclusively come fromsupernovae, the end-of-life explosions of massive stars.[27] GRB 211211A lasted 51s;[28][29] GRB 191019A (2019)[30] andGRB 230307A (2023),[31][32] with durations of around 64s and 35s respectively, have been also argued to belong to this class of long GBRs fromneutron star mergers.[33]
^Perley, D. A.; Metzger, B. D.; Granot, J.; Butler, N. R.; Sakamoto, T.; Ramirez-Ruiz, E.; Levan, A. J.; Bloom, J. S.; Miller, A. A. (2009). "GRB 080503: Implications of a Naked Short Gamma-Ray Burst Dominated by Extended Emission".The Astrophysical Journal.696 (2):1871–1885.arXiv:0811.1044.Bibcode:2009ApJ...696.1871P.doi:10.1088/0004-637X/696/2/1871.S2CID15196669.
