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Short description: Neutron star merger
Artist's impression of neutron stars merging, producing gravitational waves and resulting in a kilonova
Kilonova illustration

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.[1] 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.[2] The measured high sphericity of the kilonova AT2017gfo at early epochs was deduced from the blackbody nature of its spectrum.[3][4]


File:Neutron star merger animation ending with kilonova explosion.webm

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 is ​110 to ​1100 the brightness of a typical supernova, the self-detonation of a massive star.[5] The term kilonova was later introduced by Metzger et al. in 2010[6] 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.[7]

This artist's impression shows a kilonova produced by two colliding neutron stars.

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)[8] and demonstrated that the source was a binary neutron star merger.[9] 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.[10] Observations of AT 2017gfo confirmed that it was the first secure case of a kilonova.[11] Spectral modelling of AT2017gfo identified the r-process element Strontium and Yttrium which conclusively ties the formation of heavy elements to neutron-star mergers.[12][13] Further modelling showed the ejected fireball of heavy elements was highly spherical in early epochs.[3][14]


The inspiral and merging of two compact objects are a strong source of gravitational waves (GW).[6] The basic model for thermal transients from neutron star mergers was introduced by Li-Xin Li and Bohdan 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 dubbed kilonova.[15]


First kilonova observations by the Hubble Space Telescope[16]

A first observational suggestion of a kilonova came in 2008 following the gamma-ray burst GRB 080503,[17] 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 the short-duration gamma-ray burst GRB 130603B, where the faint infrared emission from the distant kilonova was detected using the Hubble Space Telescope.[7]

In October 2017, astronomers reported that observations of AT 2017gfo showed that it was the first secure case of a kilonova following a merger of two neutron stars.[11]

Fading kilonova in GRB160821B seen by the Hubble Space Telescope.

In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be analogous to the historic GW170817. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, 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.[18][19][20][21] In retrospect, GRB 160821B is now believed to be another gamma-ray burst event followed by a kilonova, by its resemblance of its data to AT2017gfo.[22]

A kilonova was found in the long gamma-ray burst GRB 211211A, discovered in December 2021 by Swift’s Burst Alert Telescope (BAT) and the Fermi Gamma-ray Burst Monitor (GBM).[23][24] This discovery challenges the prevailing theory that long GRBs exclusively come from supernovae, the end-of-life explosions of massive stars.[25] GRB 211211A lasted 51s,[26][27] GRB 191019A (2019)[28] and GRB 230307A (2023).[29][30] with around 64s and 35s respectively have been also argued to belong to this class of long GBRs from neutron star mergers.[31]

In 2023, GRB 230307A was observed and associated with tellurium and lanthanides.[32]

See also


  1. 1.0 1.1 1.2 Li, L.-X.; Paczyński, B.; Fruchter, A. S.; Hjorth, J.; Hounsell, R. A.; Wiersema, K.; Tunnicliffe, R. (1998). "Transient Events from Neutron Star Mergers". The Astrophysical Journal 507 (1): L59–L62. doi:10.1086/311680. Bibcode1998ApJ...507L..59L. 
  2. Rosswog, Stephan (2015-04-01). "The multi-messenger picture of compact binary mergers". International Journal of Modern Physics D 24 (5): 1530012–1530052. doi:10.1142/S0218271815300128. ISSN 0218-2718. Bibcode2015IJMPD..2430012R. 
  3. 3.0 3.1 Sneppen, Albert; Watson, Darach; Bauswein, Andreas; Just, Oliver; Kotak, Rubina; Nakar, Ehud; Poznanski, Dovi; Sim, Stuart (February 2023). "Spherical symmetry in the kilonova AT2017gfo/GW170817" (in en). Nature 614 (7948): 436–439. doi:10.1038/s41586-022-05616-x. ISSN 1476-4687. PMID 36792736. 
  4. Sneppen, Albert (2023-09-01). "On the Blackbody Spectrum of Kilonovae". The Astrophysical Journal 955 (1): 44. doi:10.3847/1538-4357/acf200. ISSN 0004-637X. 
  5. "Hubble captures infrared glow of a kilonova blast". 5 August 2013. 
  6. 6.0 6.1 Metzger, B. D.; Martínez-Pinedo, G.; Darbha, S.; Quataert, E.; Arcones, A.; Kasen, D.; Thomas, R.; Nugent, P. et al. (August 2010). "Electromagnetic counterparts of compact object mergers powered by the radioactive decay of r-process nuclei". Monthly Notices of the Royal Astronomical Society 406 (4): 2650. doi:10.1111/j.1365-2966.2010.16864.x. Bibcode2010MNRAS.406.2650M. 
  7. 7.0 7.1 Tanvir, N. R.; Levan, A. J.; Fruchter, A. S.; Hjorth, J.; Hounsell, R. A.; Wiersema, K.; Tunnicliffe, R. L. (2013). "A 'kilonova' associated with the short-duration γ-ray burst GRB 130603B". Nature 500 (7464): 547–549. doi:10.1038/nature12505. PMID 23912055. Bibcode2013Natur.500..547T. 
  8. Abbott, B. P.; Abbott, R.; Abbott, T. D.; Acernese, F.; Ackley, K.; Adams, C.; Adams, T.; Addesso, P. et al. (16 October 2017). "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral". Physical Review Letters 119 (16): 161101. doi:10.1103/PhysRevLett.119.161101. PMID 29099225. Bibcode2017PhRvL.119p1101A. 
  9. Miller, M. Coleman (16 October 2017). "Gravitational waves: A golden binary". Nature News and Views (7678): 36. doi:10.1038/nature24153. Bibcode2017Natur.551...36M. 
  10. Berger, E. (16 October 2017). "Focus on the Electromagnetic Counterpart of the Neutron Star Binary Merger GW170817". Astrophysical Journal Letters. Retrieved 16 October 2017. 
  11. 11.0 11.1 Abbott, B. P.; Abbott, R.; Abbott, T. D.; Acernese, F.; Ackley, K.; Adams, C.; Adams, T.; Addesso, P. et al. (2017-10-16). "Multi-messenger Observations of a Binary Neutron Star Merger" (in en). The Astrophysical Journal 848 (2): L12. doi:10.3847/2041-8213/aa91c9. ISSN 2041-8213. Bibcode2017ApJ...848L..12A. 
  12. Watson, Darach; Hansen, Camilla J.; Selsing, Jonatan; Koch, Andreas; Malesani, Daniele B.; Andersen, Anja C.; Fynbo, Johan P. U.; Arcones, Almudena et al. (October 2019). "Identification of strontium in the merger of two neutron stars" (in en). Nature 574 (7779): 497–500. doi:10.1038/s41586-019-1676-3. ISSN 1476-4687. PMID 31645733. 
  13. Sneppen, Albert; Watson, Darach (2023-07-01). "Discovery of a 760 nm P Cygni line in AT2017gfo: Identification of yttrium in the kilonova photosphere" (in en). Astronomy & Astrophysics 675: A194. doi:10.1051/0004-6361/202346421. ISSN 0004-6361. 
  14. "What happens when two neutron stars collide? A 'perfect' explosion." (in en-US). Washington Post. ISSN 0190-8286. 
  15. Metzger, Brian D. (2019-12-16). "Kilonovae" (in en). Living Reviews in Relativity 23 (1): 1. doi:10.1007/s41114-019-0024-0. ISSN 1433-8351. PMID 31885490. PMC 6914724. Bibcode2019LRR....23....1M. 
  16. "Hubble observes source of gravitational waves for the first time". 
  17. Perley, D. A.; Metzger, B. D.; Granot, J.; Butler, N. R.; Sakamoto, T.; Ramirez-Ruiz, E.; Levan, A. J.; Bloom, J. S. et al. (2009). "GRB 080503: Implications of a Naked Short Gamma-Ray Burst Dominated by Extended Emission" (in en). The Astrophysical Journal 696 (2): 1871–1885. doi:10.1088/0004-637X/696/2/1871. Bibcode2009ApJ...696.1871P. 
  18. University of Maryland (16 October 2018). "All in the family: Kin of gravitational wave source discovered - New observations suggest that kilonovae -- immense cosmic explosions that produce silver, gold and platinum--may be more common than thought". EurekAlert!. 
  19. Troja, E. (16 October 2018). "A luminous blue kilonova and an off-axis jet from a compact binary merger at z = 0.1341". Nature Communications 9 (1): 4089. doi:10.1038/s41467-018-06558-7. PMID 30327476. Bibcode2018NatCo...9.4089T. 
  20. Mohon, Lee (16 October 2018). "GRB 150101B: A Distant Cousin to GW170817". NASA. 
  21. Wall, Mike (17 October 2018). "Powerful Cosmic Flash Is Likely Another Neutron-Star Merger". 
  22. Strickland, Ashley (2019-08-27). "This is what it looks like when an explosion creates gold in space" (in en). 
  23. Reddy, Francis (2022-10-13). "NASA's Swift, Fermi Missions Detect Exceptional Cosmic Blast". 
  24. "Kilonova Discovery Challenges our Understanding of Gamma-Ray Bursts" (in en). 2022-12-07. 
  25. Troja, Eleonora; Dichiara, Simone (21 December 2022). "Unusual, long-lasting gamma-ray burst challenges theories about these powerful cosmic explosions that make gold, uranium and other heavy metals" (in en). 
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  27. Troja, E.; Fryer, C. L.; O’Connor, B.; Ryan, G.; Dichiara, S.; Kumar, A.; Ito, N.; Gupta, R. et al. (2022-12-08). "A nearby long gamma-ray burst from a merger of compact objects" (in en). Nature 612 (7939): 228–231. doi:10.1038/s41586-022-05327-3. ISSN 0028-0836. PMID 36477127. 
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  29. "GCN - Circulars - 33410: Solar Orbiter STIX observation of GRB 230307A". 
  30. "GCN - Circulars - 33412: GRB 230307A: AGILE/MCAL detection". 
  31. Wodd, Charlie (11 December 2023). "Extra-Long Blasts Challenge Our Theories of Cosmic Cataclysms". 
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