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The merger of neutron stars momentarily creates an environment of such extreme neutron flux that the [[r-process|''r''-process]] can occur. This reaction accounts for the [[nucleosynthesis]] of around half of the isotopes in elements heavier than iron.<ref>{{cite web |last=Stromberg |first=Joseph |date=16 July 2013 |title=All the Gold in the Universe Could Come from the Collisions of Neutron Stars |url=http://www.smithsonianmag.com/science-nature/all-the-gold-in-the-universe-could-come-from-the-collisions-of-neutron-stars-13474145/?page=1 |work=[[Smithsonian (magazine)|Smithsonian]] |access-date=27 April 2014}}</ref>
The merger of neutron stars momentarily creates an environment of such extreme neutron flux that the [[r-process|''r''-process]] can occur. This reaction accounts for the [[nucleosynthesis]] of around half of the isotopes in elements heavier than iron.<ref>{{cite web |last=Stromberg |first=Joseph |date=16 July 2013 |title=All the Gold in the Universe Could Come from the Collisions of Neutron Stars |url=http://www.smithsonianmag.com/science-nature/all-the-gold-in-the-universe-could-come-from-the-collisions-of-neutron-stars-13474145/?page=1 |work=[[Smithsonian (magazine)|Smithsonian]] |access-date=27 April 2014}}</ref>


The mergers also produce [[kilonovae]],<ref>{{cite web | url=https://www.space.com/james-webb-space-telescope-fresh-gold-cosmos-kilonova-grb | title=James Webb Space Telescope finds neutron star mergers forge gold in the cosmos: 'It was thrilling' | website=[[Space.com]] | date=21 February 2024 }}</ref> which are transient sources of [[isotropic]] longer wave electromagnetic radiation due to the [[radioactive decay]] of heavy ''r''-process nuclei that are produced and ejected during the merger process.<ref name=Tanvir2013>{{Cite journal | doi = 10.1038/nature12505| pmid = 23912055| title = A "kilonova" associated with the short-duration γ-ray burst GRB 130603B| journal = Nature| volume = 500| issue = 7464| pages = 547–9| year = 2013| 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.|arxiv = 1306.4971 |bibcode = 2013Natur.500..547T | s2cid = 205235329}}</ref> Kilonovae had been discussed as a possible ''r''-process site since the reaction was first proposed in 1999, but the mechanism became widely accepted after [[Multi-messenger astronomy|multi-messenger]] event [[GW170817]] was observed in 2017.
The mergers also produce [[kilonovae]],<ref>{{Cite web |last=Lea |first=Robert |date=21 February 2024 |title=James Webb Space Telescope finds neutron star mergers forge gold in the cosmos: 'It was thrilling' |url=https://www.space.com/james-webb-space-telescope-fresh-gold-cosmos-kilonova-grb |website=[[Space.com]]}}</ref> which are transient sources of [[isotropic]] longer wave electromagnetic radiation due to the [[radioactive decay]] of heavy ''r''-process nuclei that are produced and ejected during the merger process.<ref name=Tanvir2013>{{Cite journal | doi = 10.1038/nature12505| pmid = 23912055| title = A 'kilonova' associated with the short-duration γ-ray burst GRB 130603B| journal = Nature| volume = 500| issue = 7464| pages = 547–9| year = 2013| 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.|arxiv = 1306.4971 |bibcode = 2013Natur.500..547T | s2cid = 205235329}}</ref> Kilonovae had been discussed as a possible ''r''-process site since the reaction was first proposed in 1999, but the mechanism became widely accepted after [[Multi-messenger astronomy|multi-messenger]] event [[GW170817]] was observed in 2017.


==Observed mergers==
==Observed mergers==
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|bibcode=2017ApJ...848L..12A}}</ref>
|bibcode=2017ApJ...848L..12A}}</ref>


The co-occurrence of GW170817 with GRB 170817A in both space and time strongly implies that neutron star mergers create short gamma-ray bursts. The subsequent detection of Swope Supernova Survey event 2017a (SSS17a)<ref>{{Cite journal | doi=10.3847/2041-8213/aa9116|title = The Old Host-galaxy Environment of SSS17a, the First Electromagnetic Counterpart to a Gravitational-wave Source| journal=The Astrophysical Journal| volume=848| issue=2| pages=L30|year = 2017|last1 = Pan|first1 = Y.-C.|display-authors=etal|arxiv=1710.05439|bibcode = 2017ApJ...848L..30P|s2cid = 3516168 | doi-access=free }}</ref> in the area where GW170817 and GRB 170817A were known to have occurred—and its having the expected characteristics of a [[kilonova]]—strongly imply that neutron star mergers are responsible for kilonovae as well.<ref name=kNova>''Nature Astronomy'' [https://www.nature.com/collections/gghkrvklfb (16 Oct 2017) Kilonovae, short gamma-ray bursts & neutron star mergers]</ref>
The co-occurrence of GW170817 with GRB 170817A in both space and time strongly implies that neutron star mergers create short gamma-ray bursts. The subsequent detection of Swope Supernova Survey event 2017a (SSS17a)<ref>{{Cite journal | doi=10.3847/2041-8213/aa9116|title = The Old Host-galaxy Environment of SSS17a, the First Electromagnetic Counterpart to a Gravitational-wave Source| journal=The Astrophysical Journal| volume=848| issue=2| pages=L30|year = 2017|last1 = Pan|first1 = Y.-C.|display-authors=etal|arxiv=1710.05439|bibcode = 2017ApJ...848L..30P|s2cid = 3516168 | doi-access=free }}</ref> in the area where GW170817 and GRB 170817A were known to have occurred—and its having the expected characteristics of a [[kilonova]]—strongly imply that neutron star mergers are responsible for kilonovae as well.<ref name="kNova">{{Cite web |date=16 October 2017 |title=Kilonovae, short gamma-ray bursts & neutron star mergers |url=https://www.nature.com/collections/gghkrvklfb |website=[[Nature Astronomy]] |language=en}}</ref>


In February 2018, the [[Zwicky Transient Facility]] began to track neutron star events via gravitational wave observation,<ref name= ztfApril2019 >{{cite news |url=https://www.bbc.com/news/science-environment-48137011 |date=2 May 2019 |title=Gravitational waves hunt now in overdrive |first=Roland |last=Pease |publisher=BBC News}}</ref> as evidenced by "systematic samples of [[tidal disruption event]]s".<ref name= iopDec2018 >[https://iopscience.iop.org/article/10.1088/1538-3873/aaecbe#paspaaecbes7 Eric C. Bellm, Shrinivas R. Kulkarni, Matthew J. Graham, Richard Dekany, Roger M. Smith, Reed Riddle, Frank J. Masci, George Helou, Thomas A. Prince, Scott M. Adams (2018 December 7) The Zwicky Transient Facility: System Overview, Performance, and First Results]</ref> Later that year, astronomers reported that [[GRB 150101B]], a gamma-ray burst event detected in 2015, may be directly related to GW170817 and associated with the merger of two neutron stars. 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 [[Galaxy|galaxies]], are "striking", suggesting the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova, which may be more common in the universe than previously understood, according to the researchers.<ref name="EA-20181016">{{cite press release |publisher=University of Maryland |title=All in the family: Kin of gravitational wave source discovered |url=https://www.eurekalert.org/pub_releases/2018-10/uom-ait101518.php |date=16 October 2018 |work=[[EurekAlert!]] |access-date=17 October 2018 }}</ref><ref name="NC-20181016">{{cite journal |author=Troja, E.|display-authors=etal |title=A luminous blue kilonova and an off-axis jet from a compact binary merger at z=0.1341 |date=16 October 2018 |journal=[[Nature Communications]] |volume=9 |issue=1 |page=4089 |doi=10.1038/s41467-018-06558-7 |doi-access=free |pmid=30327476 |pmc=6191439 |bibcode=2018NatCo...9.4089T |arxiv=1806.10624 }}</ref><ref name="NASA-20181016">{{cite news |last=Mohon |first=Lee |title=GRB 150101B: A Distant Cousin to GW170817 |url=https://www.nasa.gov/mission_pages/chandra/images/grb-150101b-a-distant-cousin-to-gw170817.html |date=16 October 2018 |work=[[NASA]] |access-date=17 October 2018 }}</ref><ref name="SPC-20181017">{{cite web |last=Wall |first=Mike |title=Powerful Cosmic Flash Is Likely Another Neutron-Star Merger |url=https://www.space.com/42158-another-neutron-star-crash-detected.html |date=17 October 2018 |work=[[Space.com]] |access-date=17 October 2018 }}</ref>
In February 2018, the [[Zwicky Transient Facility]] began to track neutron star events via gravitational wave observation,<ref name= ztfApril2019 >{{cite news |url=https://www.bbc.com/news/science-environment-48137011 |date=2 May 2019 |title=Gravitational waves hunt now in overdrive |first=Roland |last=Pease |publisher=BBC News}}</ref> as evidenced by "systematic samples of [[tidal disruption event]]s".<ref name="iopDec2018">{{Cite journal |last=Bellm |first=Eric C. |display-authors=etal |date=7 December 2018 |title=The Zwicky Transient Facility: System Overview, Performance, and First Results |journal=Publications of the Astronomical Society of the Pacific |volume=131 |issue=995 |pages=018002 |doi=10.1088/1538-3873/aaecbe |issn=0004-6280}}</ref> Later that year, astronomers reported that [[GRB 150101B]], a gamma-ray burst event detected in 2015, may be directly related to GW170817 and associated with the merger of two neutron stars. 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 [[Galaxy|galaxies]], are "striking", suggesting the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova, which may be more common in the universe than previously understood, according to the researchers.<ref name="EA-20181016">{{cite press release |publisher=University of Maryland |title=All in the family: Kin of gravitational wave source discovered |url=https://www.eurekalert.org/pub_releases/2018-10/uom-ait101518.php |date=16 October 2018 |work=[[EurekAlert!]] |access-date=17 October 2018 }}</ref><ref name="NC-20181016">{{cite journal |author=Troja, E.|display-authors=etal |title=A luminous blue kilonova and an off-axis jet from a compact binary merger at z=0.1341 |date=16 October 2018 |journal=[[Nature Communications]] |volume=9 |issue=1 |page=4089 |doi=10.1038/s41467-018-06558-7 |doi-access=free |pmid=30327476 |pmc=6191439 |bibcode=2018NatCo...9.4089T |arxiv=1806.10624 }}</ref><ref name="NASA-20181016">{{cite news |last=Mohon |first=Lee |title=GRB 150101B: A Distant Cousin to GW170817 |url=https://www.nasa.gov/mission_pages/chandra/images/grb-150101b-a-distant-cousin-to-gw170817.html |date=16 October 2018 |work=[[NASA]] |access-date=17 October 2018 }}</ref><ref name="SPC-20181017">{{cite web |last=Wall |first=Mike |title=Powerful Cosmic Flash Is Likely Another Neutron-Star Merger |url=https://www.space.com/42158-another-neutron-star-crash-detected.html |date=17 October 2018 |work=[[Space.com]] |access-date=17 October 2018 }}</ref>


Also in October 2018, scientists presented a new way to use information from gravitational wave events (especially those involving the merger of neutron stars like GW170817) to determine the [[Hubble's law|Hubble constant]], which establishes the rate of [[expansion of the universe]].<ref name="PHYS-20181022">{{cite web |last=Lerner |first=Louise |title=Gravitational waves could soon provide measure of universe's expansion |url=https://phys.org/news/2018-10-gravitational-universe-expansion.html |date=22 October 2018 |work=[[Phys.org]] |access-date=22 October 2018 }}</ref><ref name="NAT-20181017">{{cite journal |last1=Chen |first1=Hsin-Yu |last2=Fishbach |first2=Maya |last3=Holz |first3=Daniel E. |title=A two per cent Hubble constant measurement from standard sirens within five years |date=17 October 2018 |journal=[[Nature (journal)|Nature]] |volume=562 |issue=7728 |pages=545–547 |doi=10.1038/s41586-018-0606-0 |pmid=30333628 |arxiv=1712.06531 |bibcode=2018Natur.562..545C |s2cid=52987203 }}</ref> The two earlier methods for finding the Hubble constant—one based on [[redshift]]s and another based on the [[cosmic distance ladder]]—disagree by about 10%. This difference, the [[Hubble tension]], might be reconciled by using kilonovae as another type of [[standard candle]].<ref name= hubbleTension>Charlie Wood [https://www.quantamagazine.org/giant-arc-of-galaxies-puts-basic-cosmology-under-scrutiny-20211213/ (13 Dec 2021) Cosmologists Parry Attacks on the Vaunted Cosmological Principle]</ref>
Also in October 2018, scientists presented a new way to use information from gravitational wave events (especially those involving the merger of neutron stars like GW170817) to determine the [[Hubble's law|Hubble constant]], which establishes the rate of [[expansion of the universe]].<ref name="PHYS-20181022">{{cite web |last=Lerner |first=Louise |title=Gravitational waves could soon provide measure of universe's expansion |url=https://phys.org/news/2018-10-gravitational-universe-expansion.html |date=22 October 2018 |work=[[Phys.org]] |access-date=22 October 2018 }}</ref><ref name="NAT-20181017">{{cite journal |last1=Chen |first1=Hsin-Yu |last2=Fishbach |first2=Maya |last3=Holz |first3=Daniel E. |title=A two per cent Hubble constant measurement from standard sirens within five years |date=17 October 2018 |journal=[[Nature (journal)|Nature]] |volume=562 |issue=7728 |pages=545–547 |doi=10.1038/s41586-018-0606-0 |pmid=30333628 |arxiv=1712.06531 |bibcode=2018Natur.562..545C |s2cid=52987203 }}</ref> The two earlier methods for finding the Hubble constant—one based on [[redshift]]s and another based on the [[cosmic distance ladder]]—disagree by about 10%. This difference, the [[Hubble tension]], might be reconciled by using kilonovae as another type of [[standard candle]].<ref name="hubbleTension">{{Cite web |last=Wood |first=Charlie |date=13 December 2021 |title=Cosmologists Parry Attacks on the Vaunted Cosmological Principle |url=https://www.quantamagazine.org/giant-arc-of-galaxies-puts-basic-cosmology-under-scrutiny-20211213/ |website=[[Quanta Magazine]] |language=en}}</ref>


In April 2019, the LIGO and Virgo gravitational wave observatories announced the detection of a candidate event that is, with a probability 99.94%, the merger of two neutron stars. Despite extensive follow-up observations, no electromagnetic counterpart could be identified.<ref>{{cite web |title=Breaking: LIGO Detects Gravitational Waves From Another Neutron Star Merger |url=http://blogs.discovermagazine.com/d-brief/2019/04/25/breaking-ligo-detects-another-neutron-star-merger/ |website=D-brief |access-date=13 August 2019 |date=25 April 2019}}</ref><ref>{{cite web |title=GraceDB {{!}} |url=https://gracedb.ligo.org/superevents/S190425z/ |website=gracedb.ligo.org |access-date=13 August 2019}}</ref><ref>{{cite journal |last1=Hosseinzadeh |first1=G. |last2=Cowperthwaite |first2=P. S. |last3=Gomez |first3=S. |last4=Villar |first4=V. A. |author-link4=V. Ashley Villar |date=18 July 2019 |title=Follow-up of the Neutron Star Bearing Gravitational Wave Candidate Events S190425z and S190426c with MMT and SOAR |url=https://inspirehep.net/record/1733318 |journal=Astrophys. J. |language=en |volume=880 |issue=1 |pages=L4 |arxiv=1905.02186 |bibcode=2019ApJ...880L...4H |doi=10.3847/2041-8213/ab271c |s2cid=146121014 |doi-access=free |hdl=10150/633863}}</ref>
In April 2019, the LIGO and Virgo gravitational wave observatories announced the detection of a candidate event that is, with a probability 99.94%, the merger of two neutron stars. Despite extensive follow-up observations, no electromagnetic counterpart could be identified.<ref>{{Cite web |date=25 April 2019 |title=Breaking: LIGO Detects Gravitational Waves From Another Neutron Star Merger |url=https://www.discovermagazine.com/the-sciences/breaking-ligo-detects-gravitational-waves-from-another-neutron-star-merger |website=[[Discover (magazine)|Discover Magazine]] |language=en}}</ref><ref>{{Cite web |title=S190425z |url=https://gracedb.ligo.org/superevents/S190425z/ |access-date=13 August 2019 |website=GraceDB}}</ref><ref>{{cite journal |last1=Hosseinzadeh |first1=G. |last2=Cowperthwaite |first2=P. S. |last3=Gomez |first3=S. |last4=Villar |first4=V. A. |author-link4=V. Ashley Villar |date=18 July 2019 |title=Follow-up of the Neutron Star Bearing Gravitational Wave Candidate Events S190425z and S190426c with MMT and SOAR |url=https://inspirehep.net/record/1733318 |journal=Astrophys. J. |language=en |volume=880 |issue=1 |pages=L4 |arxiv=1905.02186 |bibcode=2019ApJ...880L...4H |doi=10.3847/2041-8213/ab271c |s2cid=146121014 |doi-access=free |hdl=10150/633863}}</ref>


In 2023, an observation of the kilonova [[GRB 230307A]] was published, including likely observations of the spectra of [[tellurium]] and [[lanthanide]] elements.<ref>{{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 |issn=0028-0836|arxiv=2307.02098 }}</ref>
In 2023, an observation of the kilonova [[GRB 230307A]] was published, including likely observations of the spectra of [[tellurium]] and [[lanthanide]] elements.<ref>{{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 |issn=0028-0836|arxiv=2307.02098 }}</ref>
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The cosmic rays emitted by a neutron star merger occurring any less than 10 [[parsec]]s from [[Earth]] would result in conclusive human extinction.<ref name=":0">{{Cite journal |last1=Perkins |first1=Haille M. L. |last2=Ellis |first2=John |last3=Fields |first3=Brian D. |last4=Hartmann |first4=Dieter H. |last5=Liu |first5=Zhenghai |last6=McLaughlin |first6=Gail C. |last7=Surman |first7=Rebecca |last8=Wang |first8=Xilu |date=2024-02-01 |title=Could a Kilonova Kill: A Threat Assessment |journal=The Astrophysical Journal |volume=961 |issue=2 |pages=170 |doi=10.3847/1538-4357/ad12b7 |doi-access=free |arxiv=2310.11627 |bibcode=2024ApJ...961..170P |issn=0004-637X |quote="We found that cosmic rays are ... potentially lethal out to ∼10 pc, similar to the typical value of 8−20 pc for [core-collapse supernovae] ... The rarity of [binary neutron star mergers] combined with a small range of lethality means that ... the mean recurrence time of lethal mergers [on Earth] is much larger than the age of the Universe."}}</ref> By comparison, for short [[Gamma-ray burst|Gamma Ray Bursts]] (sGRB) the lethal zone extends hundreds of parsecs.<ref>{{Cite journal |last1=Melott |first1=Adrian L. |last2=Thomas |first2=Brian C. |date=May 2011 |title=Astrophysical Ionizing Radiation and Earth: A Brief Review and Census of Intermittent Intense Sources |url=http://www.liebertpub.com/doi/10.1089/ast.2010.0603 |journal=Astrobiology |language=en |volume=11 |issue=4 |pages=343–361 |doi=10.1089/ast.2010.0603 |pmid=21545268 |arxiv=1102.2830 |bibcode=2011AsBio..11..343M |issn=1531-1074}}</ref> Other sources such as [[Near-Earth supernova|near-earth supernovae]] emit high-energy photons in the form of [[gamma ray]]s and [[x-ray]]s; these would destroy Earth's [[ozone layer]], exposing the population to fatal levels of [[UVB]] radiation from the [[Sun]].
The cosmic rays emitted by a neutron star merger occurring any less than 10 [[parsec]]s from [[Earth]] would result in conclusive human extinction.<ref name=":0">{{Cite journal |last1=Perkins |first1=Haille M. L. |last2=Ellis |first2=John |last3=Fields |first3=Brian D. |last4=Hartmann |first4=Dieter H. |last5=Liu |first5=Zhenghai |last6=McLaughlin |first6=Gail C. |last7=Surman |first7=Rebecca |last8=Wang |first8=Xilu |date=2024-02-01 |title=Could a Kilonova Kill: A Threat Assessment |journal=The Astrophysical Journal |volume=961 |issue=2 |pages=170 |doi=10.3847/1538-4357/ad12b7 |doi-access=free |arxiv=2310.11627 |bibcode=2024ApJ...961..170P |issn=0004-637X |quote="We found that cosmic rays are ... potentially lethal out to ∼10 pc, similar to the typical value of 8−20 pc for [core-collapse supernovae] ... The rarity of [binary neutron star mergers] combined with a small range of lethality means that ... the mean recurrence time of lethal mergers [on Earth] is much larger than the age of the Universe."}}</ref> By comparison, for short [[Gamma-ray burst|Gamma Ray Bursts]] (sGRB) the lethal zone extends hundreds of parsecs.<ref>{{Cite journal |last1=Melott |first1=Adrian L. |last2=Thomas |first2=Brian C. |date=May 2011 |title=Astrophysical Ionizing Radiation and Earth: A Brief Review and Census of Intermittent Intense Sources |url=http://www.liebertpub.com/doi/10.1089/ast.2010.0603 |journal=Astrobiology |language=en |volume=11 |issue=4 |pages=343–361 |doi=10.1089/ast.2010.0603 |pmid=21545268 |arxiv=1102.2830 |bibcode=2011AsBio..11..343M |issn=1531-1074}}</ref> Other sources such as [[Near-Earth supernova|near-earth supernovae]] emit high-energy photons in the form of [[gamma ray]]s and [[x-ray]]s; these would destroy Earth's [[ozone layer]], exposing the population to fatal levels of [[UVB]] radiation from the [[Sun]].


Compared to these, neutron star mergers are unique in that they emit multiple sources of harmful radiation, including emission from the radioactive decay of heavy elements<ref>{{Cite journal |last1=Wang 王夕露) |first1=Xilu |last2=N3AS Collaboration |last3=Vassh |first3=Nicole |last4=FIRE Collaboration |last5=Sprouse |first5=Trevor |last6=Mumpower |first6=Matthew |last7=Vogt |first7=Ramona |last8=Randrup |first8=Jorgen |last9=Surman |first9=Rebecca |date=2020-11-01 |title=MeV Gamma Rays from Fission: A Distinct Signature of Actinide Production in Neutron Star Mergers |journal=The Astrophysical Journal Letters |volume=903 |issue=1 |pages=L3 |doi=10.3847/2041-8213/abbe18 |doi-access=free |arxiv=2008.03335 |bibcode=2020ApJ...903L...3W |issn=2041-8205}}</ref> scattered by the sGRB cocoon,<ref>{{Cite journal |last1=Kisaka |first1=Shota |last2=Ioka |first2=Kunihito |last3=Kashiyama |first3=Kazumi |last4=Nakamura |first4=Takashi |date=2018-11-01 |title=Scattered Short Gamma-Ray Bursts as Electromagnetic Counterparts to Gravitational Waves and Implications of GW170817 and GRB 170817A |journal=The Astrophysical Journal |volume=867 |issue=1 |pages=39 |doi=10.3847/1538-4357/aae30a |doi-access=free |arxiv=1711.00243 |bibcode=2018ApJ...867...39K |issn=0004-637X}}</ref> the sGRB afterglow itself,<ref>{{Cite journal |last1=Makhathini |first1=S. |last2=Mooley |first2=K. P. |last3=Brightman |first3=M. |last4=Hotokezaka |first4=K. |last5=Nayana |first5=A. J. |last6=Intema |first6=H. T. |last7=Dobie |first7=D. |last8=Lenc |first8=E. |last9=Perley |first9=D. A. |last10=Fremling |first10=C. |last11=Moldòn |first11=J. |last12=Lazzati |first12=D. |last13=Kaplan |first13=D. L. |last14=Balasubramanian |first14=A. |last15=Brown |first15=I. S. |date=2021-12-01 |title=The Panchromatic Afterglow of GW170817: The Full Uniform Data Set, Modeling, Comparison with Previous Results, and Implications |journal=The Astrophysical Journal |volume=922 |issue=2 |pages=154 |doi=10.3847/1538-4357/ac1ffc |doi-access=free |arxiv=2006.02382 |bibcode=2021ApJ...922..154M |issn=0004-637X}}</ref> and [[cosmic ray]]s accelerated by the blast. In order of arrival, the photons are first after the merger, and the cosmic rays arrive hundreds to thousands of years later. {{xref|(See: [[Multi-messenger astronomy]])}} The ejected material sweeps up the interstellar medium and creates a [[Supernova remnant|supernova-remnant]]-like bubble holding a lethal dose of cosmic rays. If the Earth were to be engulfed by the remnant, these cosmic rays—like the gamma rays—would deplete the ozone and could interact with the atmosphere, yielding weakly-interacting [[muon]]s. The flux density of these generated particles would be sufficient to sterilize the planet, penetrating even deep into caves and underwater. The danger to life lies in the particles' ability to disrupt DNA, causing birth defects and mutations.<ref>{{Cite journal |last1=Dar |first1=Arnon |last2=Laor |first2=Ari |last3=Shaviv |first3=Nir J. |date=1998-06-29 |title=Life Extinctions by Cosmic Ray Jets |url=https://link.aps.org/doi/10.1103/PhysRevLett.80.5813 |journal=Physical Review Letters |language=en |volume=80 |issue=26 |pages=5813–5816 |doi=10.1103/PhysRevLett.80.5813 |arxiv=astro-ph/9705008 |bibcode=1998PhRvL..80.5813D |issn=0031-9007}}</ref><ref>{{Cite journal |last=Juckett |first=David A. |date=November 2009 |title=A 17-year oscillation in cancer mortality birth cohorts on three continents – synchrony to cosmic ray modulations one generation earlier |url=http://link.springer.com/10.1007/s00484-009-0237-0 |journal=International Journal of Biometeorology |language=en |volume=53 |issue=6 |pages=487–499 |doi=10.1007/s00484-009-0237-0 |pmid=19506913 |bibcode=2009IJBm...53..487J |issn=0020-7128}}</ref>
Compared to these, neutron star mergers are unique in that they emit multiple sources of harmful radiation, including emission from the radioactive decay of heavy elements<ref>{{Cite journal |last=Wang |first=Xilu |date=November 2020 |title=MeV Gamma Rays from Fission: A Distinct Signature of Actinide Production in Neutron Star Mergers |journal=[[The Astrophysical Journal]] |volume=903 |issue=1 |pages=L3 |arxiv=2008.03335 |bibcode=2020ApJ...903L...3W |doi=10.48550/arXiv.2008.03335 |issn=0004-637X |doi-access=free |collaboration=N3AS and FIRE Collaboration}}</ref> scattered by the sGRB cocoon,<ref>{{Cite journal |last1=Kisaka |first1=Shota |last2=Ioka |first2=Kunihito |last3=Kashiyama |first3=Kazumi |last4=Nakamura |first4=Takashi |date=2018-11-01 |title=Scattered Short Gamma-Ray Bursts as Electromagnetic Counterparts to Gravitational Waves and Implications of GW170817 and GRB 170817A |journal=The Astrophysical Journal |volume=867 |issue=1 |pages=39 |doi=10.3847/1538-4357/aae30a |doi-access=free |arxiv=1711.00243 |bibcode=2018ApJ...867...39K |issn=0004-637X}}</ref> the sGRB afterglow itself,<ref>{{Cite journal |last1=Makhathini |first1=S. |last2=Mooley |first2=K. P. |last3=Brightman |first3=M. |last4=Hotokezaka |first4=K. |last5=Nayana |first5=A. J. |last6=Intema |first6=H. T. |last7=Dobie |first7=D. |last8=Lenc |first8=E. |last9=Perley |first9=D. A. |last10=Fremling |first10=C. |last11=Moldòn |first11=J. |last12=Lazzati |first12=D. |last13=Kaplan |first13=D. L. |last14=Balasubramanian |first14=A. |last15=Brown |first15=I. S. |date=2021-12-01 |title=The Panchromatic Afterglow of GW170817: The Full Uniform Data Set, Modeling, Comparison with Previous Results, and Implications |journal=The Astrophysical Journal |volume=922 |issue=2 |pages=154 |doi=10.3847/1538-4357/ac1ffc |doi-access=free |arxiv=2006.02382 |bibcode=2021ApJ...922..154M |issn=0004-637X}}</ref> and [[cosmic ray]]s accelerated by the blast. In order of arrival, the photons are first after the merger, and the cosmic rays arrive hundreds to thousands of years later. {{xref|(See: [[Multi-messenger astronomy]])}} The ejected material sweeps up the interstellar medium and creates a [[Supernova remnant|supernova-remnant]]-like bubble holding a lethal dose of cosmic rays. If the Earth were to be engulfed by the remnant, these cosmic rays—like the gamma rays—would deplete the ozone and could interact with the atmosphere, yielding weakly-interacting [[muon]]s. The flux density of these generated particles would be sufficient to sterilize the planet, penetrating even deep into caves and underwater. The danger to life lies in the particles' ability to disrupt DNA, causing birth defects and mutations.<ref>{{Cite journal |last1=Dar |first1=Arnon |last2=Laor |first2=Ari |last3=Shaviv |first3=Nir J. |date=1998-06-29 |title=Life Extinctions by Cosmic Ray Jets |url=https://link.aps.org/doi/10.1103/PhysRevLett.80.5813 |journal=Physical Review Letters |language=en |volume=80 |issue=26 |pages=5813–5816 |doi=10.1103/PhysRevLett.80.5813 |arxiv=astro-ph/9705008 |bibcode=1998PhRvL..80.5813D |issn=0031-9007}}</ref><ref>{{Cite journal |last=Juckett |first=David A. |date=November 2009 |title=A 17-year oscillation in cancer mortality birth cohorts on three continents – synchrony to cosmic ray modulations one generation earlier |url=http://link.springer.com/10.1007/s00484-009-0237-0 |journal=International Journal of Biometeorology |language=en |volume=53 |issue=6 |pages=487–499 |doi=10.1007/s00484-009-0237-0 |pmid=19506913 |bibcode=2009IJBm...53..487J |issn=0020-7128}}</ref>


Relative to supernovae, binary neutron star (BNS) mergers influence a similar volume of space, but they are much rarer and have a stronger dependence on the orientation of the event with respect to Earth. Accordingly, the overall threat of a BNS event to human extinction is extremely low.<ref name=":0" />
Relative to supernovae, binary neutron star (BNS) mergers influence a similar volume of space, but they are much rarer and have a stronger dependence on the orientation of the event with respect to Earth. Accordingly, the overall threat of a BNS event to human extinction is extremely low.<ref name=":0" />

Revision as of 19:55, 7 December 2024

"Neutron star collision" redirects here. For the Muse song, see Neutron Star Collision (Love Is Forever).
Artist's impression of neutron stars merging, producing gravitational waves and resulting in a kilonova
Artist's impression of neutron stars merging, producing gravitational waves and resulting in a kilonova

A neutron star merger is the stellar collision of neutron stars. When two neutron stars fall into mutual orbit, they gradually spiral inward due to the loss of energy emitted as gravitational radiation.[1] When they finally meet, their merger leads to the formation of either a more massive neutron star, or—if the mass of the remnant exceeds the Tolman–Oppenheimer–Volkoff limit—a black hole. The merger can create a magnetic field that is trillions of times stronger than that of Earth in a matter of one or two milliseconds.[2] These events are believed to create short gamma-ray bursts.[3]

The merger of neutron stars momentarily creates an environment of such extreme neutron flux that the r-process can occur. This reaction accounts for the nucleosynthesis of around half of the isotopes in elements heavier than iron.[4]

The mergers also produce kilonovae,[5] which are transient sources of isotropic longer wave electromagnetic radiation due to the radioactive decay of heavy r-process nuclei that are produced and ejected during the merger process.[6] Kilonovae had been discussed as a possible r-process site since the reaction was first proposed in 1999, but the mechanism became widely accepted after multi-messenger event GW170817 was observed in 2017.

Observed mergers

17 August 2017: Gravitational wave (GW170817) detected from merger of two neutron stars (00:23 video; artist concept).

On 17 August 2017, the LIGO and Virgo interferometers observed GW170817,[7] a gravitational wave associated with the merger of two neutron stars in NGC 4993, an elliptical galaxy in the constellation Hydra about 140 million light years away.[8] GW170817 co-occurred with a short (roughly 2-second long) gamma-ray burst, GRB 170817A, first detected 1.7 seconds after the GW merger signal, and a visible light observational event first observed 11 hours afterwards, SSS17a.[9][10][11][12][13]

The co-occurrence of GW170817 with GRB 170817A in both space and time strongly implies that neutron star mergers create short gamma-ray bursts. The subsequent detection of Swope Supernova Survey event 2017a (SSS17a)[14] in the area where GW170817 and GRB 170817A were known to have occurred—and its having the expected characteristics of a kilonova—strongly imply that neutron star mergers are responsible for kilonovae as well.[15]

In February 2018, the Zwicky Transient Facility began to track neutron star events via gravitational wave observation,[16] as evidenced by "systematic samples of tidal disruption events".[17] Later that year, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be directly related to GW170817 and associated with the merger of two neutron stars. 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 "striking", suggesting the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova, which may be more common in the universe than previously understood, according to the researchers.[18][19][20][21]

Also in October 2018, scientists presented a new way to use information from gravitational wave events (especially those involving the merger of neutron stars like GW170817) to determine the Hubble constant, which establishes the rate of expansion of the universe.[22][23] The two earlier methods for finding the Hubble constant—one based on redshifts and another based on the cosmic distance ladder—disagree by about 10%. This difference, the Hubble tension, might be reconciled by using kilonovae as another type of standard candle.[24]

In April 2019, the LIGO and Virgo gravitational wave observatories announced the detection of a candidate event that is, with a probability 99.94%, the merger of two neutron stars. Despite extensive follow-up observations, no electromagnetic counterpart could be identified.[25][26][27]

In 2023, an observation of the kilonova GRB 230307A was published, including likely observations of the spectra of tellurium and lanthanide elements.[28]

XT2 (magnetar)

"XT2" redirects here. For the camera, see Fujifilm X-T2.

In 2019, analysis of data from the Chandra X-ray Observatory revealed another binary neutron star merger at a distance of 6.6 billion light years, an x-ray signal called XT2. The merger produced a magnetar; its emissions could be detected for several hours.[29]

Effect on Earth

The cosmic rays emitted by a neutron star merger occurring any less than 10 parsecs from Earth would result in conclusive human extinction.[30] By comparison, for short Gamma Ray Bursts (sGRB) the lethal zone extends hundreds of parsecs.[31] Other sources such as near-earth supernovae emit high-energy photons in the form of gamma rays and x-rays; these would destroy Earth's ozone layer, exposing the population to fatal levels of UVB radiation from the Sun.

Compared to these, neutron star mergers are unique in that they emit multiple sources of harmful radiation, including emission from the radioactive decay of heavy elements[32] scattered by the sGRB cocoon,[33] the sGRB afterglow itself,[34] and cosmic rays accelerated by the blast. In order of arrival, the photons are first after the merger, and the cosmic rays arrive hundreds to thousands of years later. (See: Multi-messenger astronomy) The ejected material sweeps up the interstellar medium and creates a supernova-remnant-like bubble holding a lethal dose of cosmic rays. If the Earth were to be engulfed by the remnant, these cosmic rays—like the gamma rays—would deplete the ozone and could interact with the atmosphere, yielding weakly-interacting muons. The flux density of these generated particles would be sufficient to sterilize the planet, penetrating even deep into caves and underwater. The danger to life lies in the particles' ability to disrupt DNA, causing birth defects and mutations.[35][36]

Relative to supernovae, binary neutron star (BNS) mergers influence a similar volume of space, but they are much rarer and have a stronger dependence on the orientation of the event with respect to Earth. Accordingly, the overall threat of a BNS event to human extinction is extremely low.[30]

Distribution of Heavy Metals

Neutron star mergers are rare, so most stars will form out of gas clouds which have few r-process metals. Our own solar system, however, did form from a gas cloud enriched with heavy metals.[citation needed] This suggests that metals heavier than iron, such as the platinum group metals, the rare earth elements, and the radioactive elements will be rarer in most solar systems as compared to our own.

See also

References

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  31. ^ Melott, Adrian L.; Thomas, Brian C. (May 2011). "Astrophysical Ionizing Radiation and Earth: A Brief Review and Census of Intermittent Intense Sources". Astrobiology. 11 (4): 343–361. arXiv:1102.2830. Bibcode:2011AsBio..11..343M. doi:10.1089/ast.2010.0603. ISSN 1531-1074. PMID 21545268.
  32. ^ Wang, Xilu; et al. (N3AS and FIRE Collaboration) (November 2020). "MeV Gamma Rays from Fission: A Distinct Signature of Actinide Production in Neutron Star Mergers". The Astrophysical Journal. 903 (1): L3. arXiv:2008.03335. Bibcode:2020ApJ...903L...3W. doi:10.48550/arXiv.2008.03335. ISSN 0004-637X.
  33. ^ Kisaka, Shota; Ioka, Kunihito; Kashiyama, Kazumi; Nakamura, Takashi (1 November 2018). "Scattered Short Gamma-Ray Bursts as Electromagnetic Counterparts to Gravitational Waves and Implications of GW170817 and GRB 170817A". The Astrophysical Journal. 867 (1): 39. arXiv:1711.00243. Bibcode:2018ApJ...867...39K. doi:10.3847/1538-4357/aae30a. ISSN 0004-637X.
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