Kilonova

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. The radioactive glow arising from the merger ejecta was originally called mini-supernova, as it is {{frac|1|10}} to {{frac|1|100}} the brightness of a typical supernova, the self-detonation of a massive star.{{cite web |date=5 August 2013 |title=Hubble captures infrared glow of a kilonova blast |url=http://www.spacetelescope.org/images/opo1329a/ |access-date=28 February 2018 |publisher=spacetelescope.org}} 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 on June 3, 2013 as short gamma-ray burst GRB 130603B by instruments on board the Swift Gamma-Ray Burst Explorer and KONUS/WIND spacecraft, and then imaged by the Hubble Space Telescope 9 and 30 days later.{{Cite journal |last1=Tanvir |first1=N. R. |last2=Levan |first2=A. J. |last3=Fruchter |first3=A. S. |last4=Hjorth |first4=J. |last5=Hounsell |first5=R. A. |last6=Wiersema |first6=K. |last7=Tunnicliffe |first7=R. L. |year=2013 |title=A 'kilonova' associated with the short-duration γ-ray burst GRB 130603B |journal=Nature |volume=500 |issue=7464 |pages=547–549 |arxiv=1306.4971 |bibcode=2013Natur.500..547T |doi=10.1038/nature12505 |pmid=23912055 |s2cid=205235329}}

File:Noirlab2228a - Artist's impression of colliding neutron stars.jpg

On October 16, 2017, the LIGO and Virgo collaborations announced the detection of GW170817,{{cite journal |last1=Abbott |first1=B. P. |last2=Abbott |first2=R. |last3=Abbott |first3=T. D. |last4=Acernese |first4=F. |last5=Ackley |first5=K. |last6=Adams |first6=C. |last7=Adams |first7=T. |last8=Addesso |first8=P. |last9=Adhikari |first9=R. X. |last10=Adya |first10=V. B. |date=16 October 2017 |title=GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral |journal=Physical Review Letters |volume=119 |issue=16 |pages=161101 |arxiv=1710.05832 |bibcode=2017PhRvL.119p1101A |doi=10.1103/PhysRevLett.119.161101 |pmid=29099225 |collaboration=LIGO Scientific Collaboration & Virgo Collaboration |s2cid=217163611}} the first gravitational wave (GW) shown to have originated from the binary merger of neutron stars.{{cite journal |last1=Miller |first1=M. Coleman |date=16 October 2017 |title=Gravitational waves: A golden binary |journal=Nature |volume=News and Views |issue=7678 |pages=36 |bibcode=2017Natur.551...36M |doi=10.1038/nature24153 |doi-access=free}} 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 {{nowrap|(GRB 170817A)}}, and then after several hours, a longer lasting astronomical transient (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 galaxy NGC 4993.{{Cite journal |last1=Berger |first1=E. |date=16 October 2017 |title=Focus on the Electromagnetic Counterpart of the Neutron Star Binary Merger GW170817 |url=http://iopscience.iop.org/journal/2041-8205/page/Focus_on_GW170817 |journal=Astrophysical Journal Letters |access-date=16 October 2017}} Observations of AT 2017gfo confirmed that it was the first conclusive observation of a kilonova.{{Cite journal |last1=Abbott |first1=B. P. |last2=Abbott |first2=R. |last3=Abbott |first3=T. D. |last4=Acernese |first4=F. |last5=Ackley |first5=K. |last6=Adams |first6=C. |last7=Adams |first7=T. |last8=Addesso |first8=P. |last9=Adhikari |first9=R. X. |last10=Adya |first10=V. B. |last11=Affeldt |first11=C. |last12=Afrough |first12=M. |last13=Agarwal |first13=B. |last14=Agathos |first14=M. |last15=Agatsuma |first15=K. |date=2017-10-16 |title=Multi-messenger Observations of a Binary Neutron Star Merger |journal=The Astrophysical Journal |language=en |volume=848 |issue=2 |pages=L12 |doi=10.3847/2041-8213/aa91c9 |arxiv=1710.05833 |bibcode=2017ApJ...848L..12A |s2cid=217162243 |issn=2041-8213 |doi-access=free }} Spectral modelling of AT2017gfo identified the r-process elements strontium and yttrium, which conclusively ties the formation of heavy elements to neutron-star mergers.{{Cite journal |last1=Watson |first1=Darach |last2=Hansen |first2=Camilla J. |last3=Selsing |first3=Jonatan |last4=Koch |first4=Andreas |last5=Malesani |first5=Daniele B. |last6=Andersen |first6=Anja C. |last7=Fynbo |first7=Johan P. U. |last8=Arcones |first8=Almudena |last9=Bauswein |first9=Andreas |last10=Covino |first10=Stefano |last11=Grado |first11=Aniello |last12=Heintz |first12=Kasper E. |last13=Hunt |first13=Leslie |last14=Kouveliotou |first14=Chryssa |last15=Leloudas |first15=Giorgos |date=October 2019 |title=Identification of strontium in the merger of two neutron stars |url=https://www.nature.com/articles/s41586-019-1676-3 |journal=Nature |language=en |volume=574 |issue=7779 |pages=497–500 |doi=10.1038/s41586-019-1676-3 |pmid=31645733 |issn=1476-4687|arxiv=1910.10510 |bibcode=2019Natur.574..497W |s2cid=204837882 }}{{Cite journal |last1=Sneppen |first1=Albert |last2=Watson |first2=Darach |date=2023-07-01 |title=Discovery of a 760 nm P Cygni line in AT2017gfo: Identification of yttrium in the kilonova photosphere |url=https://www.aanda.org/articles/aa/abs/2023/07/aa46421-23/aa46421-23.html |journal=Astronomy & Astrophysics |language=en |volume=675 |pages=A194 |doi=10.1051/0004-6361/202346421 |issn=0004-6361|arxiv=2306.14942 |bibcode=2023A&A...675A.194S }} Further modelling showed the ejected fireball of heavy elements was highly spherical in early epochs.{{Cite journal |last1=Sneppen |first1=Albert |last2=Watson |first2=Darach |last3=Bauswein |first3=Andreas |last4=Just |first4=Oliver |last5=Kotak |first5=Rubina |last6=Nakar |first6=Ehud |last7=Poznanski |first7=Dovi |last8=Sim |first8=Stuart |date=February 2023 |title=Spherical symmetry in the kilonova AT2017gfo/GW170817 |url=https://www.nature.com/articles/s41586-022-05616-x |journal=Nature |language=en |volume=614 |issue=7948 |pages=436–439 |doi=10.1038/s41586-022-05616-x |issn=1476-4687 |pmid=36792736 |arxiv=2302.06621 |bibcode=2023Natur.614..436S |s2cid=256846834 }}{{Cite news |title=What happens when two neutron stars collide? A 'perfect' explosion. |language=en-US |newspaper=Washington Post |url=https://www.washingtonpost.com/science/2023/02/16/kilonova-perfect-explosion-black-hole/ |access-date=2023-02-18 |issn=0190-8286}} Some researchers have suggested that "thanks to this work, astronomers could use kilonovae as a standard 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."{{cite web | url=https://www.universetoday.com/160165/when-neutron-stars-collide-the-explosion-is-perfectly-spherical/#more-160165 | title=When Neutron Stars Collide, the Explosion is Perfectly Spherical | date=17 February 2023 }}

The inspiral and merging of two compact objects are a strong source of gravitational waves (GW).{{cite journal |title=Electromagnetic counterparts of compact object mergers powered by the radioactive decay of r-process nuclei |journal=Monthly Notices of the Royal Astronomical Society |volume=406 |issue=4 |page=2650 |date=August 2010 |doi=10.1111/j.1365-2966.2010.16864.x |bibcode=2010MNRAS.406.2650M |arxiv=1001.5029 |last1=Metzger |first1=B. D. |last2=Martínez-Pinedo |first2=G. |last3=Darbha |first3=S. |last4=Quataert |first4=E. |last5=Arcones |first5=A.|author5-link=Almudena Arcones |last6=Kasen |first6=D. |last7=Thomas |first7=R. |last8=Nugent |first8=P. |last9=Panov |first9=I. V. |last10=Zinner |first10=N. T.|doi-access=free |s2cid=118863104 }} The basic model for thermal transients from neutron star mergers was introduced by Li-Xin Li and Bohdan Paczyński in 1998.{{cite journal |title=Transient Events from Neutron Star Mergers |journal=The Astrophysical Journal |date=1998 |volume=507 |issue=1 |page=L59–L62 |doi=10.1086/311680 |arxiv=astro-ph/9807272 |bibcode=1998ApJ...507L..59L|last1=Li |first1=L.-X. |last2=Paczyński |first2=B. |last3=Fruchter |first3=A. S. |last4=Hjorth |first4=J. |last5=Hounsell |first5=R. A. |last6=Wiersema |first6=K. |last7=Tunnicliffe |first7=R. |s2cid=3091361 }} 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.{{Cite journal|last=Metzger|first=Brian D.|date=2019-12-16|title=Kilonovae|url=https://doi.org/10.1007/s41114-019-0024-0|journal=Living Reviews in Relativity|language=en|volume=23|issue=1|pages=1|doi=10.1007/s41114-019-0024-0|arxiv=1910.01617 |issn=1433-8351|pmc=6914724|pmid=31885490|bibcode=2019LRR....23....1M }}

File:Hubble observes first kilonova.jpg{{cite web |title=Hubble observes source of gravitational waves for the first time |url=https://www.spacetelescope.org/news/heic1717/ |website=www.spacetelescope.org |access-date=18 October 2017}}]]

A first observational suggestion of a kilonova came in 2008 following the gamma-ray burst GRB 080503,{{Cite journal |last1=Perley |first1=D. A. |last2=Metzger |first2=B. D. |last3=Granot |first3=J. |last4=Butler |first4=N. R. |last5=Sakamoto |first5=T. |last6=Ramirez-Ruiz |first6=E. |last7=Levan |first7=A. J. |last8=Bloom |first8=J. S. |last9=Miller |first9=A. A. |year=2009 |title=GRB 080503: Implications of a Naked Short Gamma-Ray Burst Dominated by Extended Emission |journal=The Astrophysical Journal |language=en |volume=696 |issue=2 |pages=1871–1885 |doi=10.1088/0004-637X/696/2/1871 |arxiv=0811.1044 |bibcode=2009ApJ...696.1871P|s2cid=15196669 }} 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.

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

File:GRB160821B.gif.]]

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.{{cite news |author=University of Maryland |author-link=University of Maryland |date=16 October 2018 |title=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 |work=EurekAlert! |url=https://www.eurekalert.org/pub_releases/2018-10/uom-ait101518.php |access-date=17 October 2018}}{{cite journal |author=Troja, E. |display-authors=etal |date=16 October 2018 |title=A luminous blue kilonova and an off-axis jet from a compact binary merger at z = 0.1341 |journal=Nature Communications |volume=9 |issue=1 |pages=4089 |arxiv=1806.10624 |bibcode=2018NatCo...9.4089T |doi=10.1038/s41467-018-06558-7 |pmc=6191439 |pmid=30327476}}{{cite news |last=Mohon |first=Lee |date=16 October 2018 |title=GRB 150101B: A Distant Cousin to GW170817 |work=NASA |url=https://www.nasa.gov/mission_pages/chandra/images/grb-150101b-a-distant-cousin-to-gw170817.html |access-date=17 October 2018}}{{cite web |last=Wall |first=Mike |date=17 October 2018 |title=Powerful Cosmic Flash Is Likely Another Neutron-Star Merger |url=https://www.space.com/42158-another-neutron-star-crash-detected.html |access-date=17 October 2018 |work=Space.com}} 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 to AT2017gfo.{{Cite web |last=Strickland |first=Ashley |date=2019-08-27 |title=This is what it looks like when an explosion creates gold in space |url=https://www.cnn.com/2019/08/27/world/kilanova-gold-2016-scn-trnd/index.html |access-date=2022-12-11 |website=CNN |language=en}}

A kilonova was also thought to have caused 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).{{Cite web |last=Reddy |first=Francis |date=2022-10-13 |title=NASA's Swift, Fermi Missions Detect Exceptional Cosmic Blast |url=http://www.nasa.gov/feature/goddard/2022/nasa-s-swift-fermi-missions-detect-exceptional-cosmic-blast |access-date=2022-12-11 |website=NASA}}{{Cite web |date=2022-12-07 |title=Kilonova Discovery Challenges our Understanding of Gamma-Ray Bursts |url=https://www.gemini.edu/pr/kilonova-discovery-challenges-our-understanding-gamma-ray-bursts |access-date=2022-12-11 |website=Gemini Observatory |language=en}} These discoveries challenge the formerly prevailing theory that long GRBs exclusively come from supernovae, the end-of-life explosions of massive stars.{{Cite web |last1=Troja |first1=Eleonora |last2=Dichiara |first2=Simone |title=Unusual, long-lasting gamma-ray burst challenges theories about these powerful cosmic explosions that make gold, uranium and other heavy metals |url=http://theconversation.com/unusual-long-lasting-gamma-ray-burst-challenges-theories-about-these-powerful-cosmic-explosions-that-make-gold-uranium-and-other-heavy-metals-196859 |access-date=2022-12-27 |website=The Conversation |date=21 December 2022 |language=en}} GRB 211211A lasted 51s;{{Cite journal |last1=Rastinejad |first1=Jillian C. |last2=Gompertz |first2=Benjamin P. |last3=Levan |first3=Andrew J. |last4=Fong |first4=Wen-fai |last5=Nicholl |first5=Matt |last6=Lamb |first6=Gavin P. |last7=Malesani |first7=Daniele B. |last8=Nugent |first8=Anya E. |last9=Oates |first9=Samantha R. |last10=Tanvir |first10=Nial R. |last11=de Ugarte Postigo |first11=Antonio |last12=Kilpatrick |first12=Charles D. |last13=Moore |first13=Christopher J. |last14=Metzger |first14=Brian D. |last15=Ravasio |first15=Maria Edvige |date=2022-12-08 |title=A kilonova following a long-duration gamma-ray burst at 350 Mpc |url=https://www.nature.com/articles/s41586-022-05390-w |journal=Nature |language=en |volume=612 |issue=7939 |pages=223–227 |doi=10.1038/s41586-022-05390-w |pmid=36477128 |issn=0028-0836|arxiv=2204.10864 |bibcode=2022Natur.612..223R |s2cid=248376822 }}{{Cite journal |last1=Troja |first1=E. |last2=Fryer |first2=C. L. |last3=O’Connor |first3=B. |last4=Ryan |first4=G. |last5=Dichiara |first5=S. |last6=Kumar |first6=A. |last7=Ito |first7=N. |last8=Gupta |first8=R. |last9=Wollaeger |first9=R. T. |last10=Norris |first10=J. P. |last11=Kawai |first11=N. |last12=Butler |first12=N. R. |last13=Aryan |first13=A. |last14=Misra |first14=K. |last15=Hosokawa |first15=R. |date=2022-12-08 |title=A nearby long gamma-ray burst from a merger of compact objects |journal=Nature |language=en |volume=612 |issue=7939 |pages=228–231 |doi=10.1038/s41586-022-05327-3 |issn=0028-0836 |pmc=9729102 |pmid=36477127|arxiv=2209.03363 |bibcode=2022Natur.612..228T }} GRB 191019A (2019){{Cite journal |last1=Levan |first1=Andrew J. |last2=Malesani |first2=Daniele B. |last3=Gompertz |first3=Benjamin P. |last4=Nugent |first4=Anya E. |last5=Nicholl |first5=Matt |last6=Oates |first6=Samantha R. |last7=Perley |first7=Daniel A. |last8=Rastinejad |first8=Jillian |last9=Metzger |first9=Brian D. |last10=Schulze |first10=Steve |last11=Stanway |first11=Elizabeth R. |last12=Inkenhaag |first12=Anne |last13=Zafar |first13=Tayyaba |last14=Agüí Fernández |first14=J. Feliciano |last15=Chrimes |first15=Ashley A. |date=2023-06-22 |title=A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy |url=https://www.nature.com/articles/s41550-023-01998-8 |journal=Nature Astronomy |language=en |volume=7 |issue=8 |pages=976–985 |doi=10.1038/s41550-023-01998-8 |issn=2397-3366|arxiv=2303.12912 |bibcode=2023NatAs...7..976L |s2cid=257687190 }} and GRB 230307A (2023),{{cite web |title=GCN - Circulars - 33410: Solar Orbiter STIX observation of GRB 230307A |url=https://gcn.nasa.gov/circulars/33410}}{{cite web |title=GCN - Circulars - 33412: GRB 230307A: AGILE/MCAL detection |url=https://gcn.nasa.gov/circulars/33412}} with durations of around 64s and 35s respectively, have been also argued to belong to this class of long GBRs from neutron star mergers.{{Cite web |last=Wodd |first=Charlie |date=11 December 2023 |title=Extra-Long Blasts Challenge Our Theories of Cosmic Cataclysms |url=https://www.quantamagazine.org/extra-long-blasts-challenge-our-theories-of-cosmic-cataclysms-20231211/ |website=Quanta magazine}}https://www.space.com/the-universe/the-most-powerful-explosions-in-the-universe-could-reveal-where-gold-comes-from

In 2023, GRB 230307A was observed and associated with tellurium and lanthanides.{{Cite journal |last1=Levan |first1=Andrew |last2=Gompertz |first2=Benjamin P. |last3=Salafia |first3=Om Sharan |last4=Bulla |first4=Mattia |last5=Burns |first5=Eric |last6=Hotokezaka |first6=Kenta |last7=Izzo |first7=Luca |last8=Lamb |first8=Gavin P. |last9=Malesani |first9=Daniele B. |last10=Oates |first10=Samantha R. |last11=Ravasio |first11=Maria Edvige |last12=Rouco Escorial |first12=Alicia |last13=Schneider |first13=Benjamin |last14=Sarin |first14=Nikhil |last15=Schulze |first15=Steve |date=2023-10-25 |title=Heavy element production in a compact object merger observed by JWST |journal=Nature |volume=626 |issue=8000 |pages=737–741 |language=en |doi=10.1038/s41586-023-06759-1 |pmid=37879361 |pmc=10881391 |s2cid=264489953 |issn=0028-0836|arxiv=2307.02098 }}

• Hypernova
• Nova
• R-process
• Supernova
• Supernova impostor

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{{Neutron star}}

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