GW170817#Scientific importance

{{Quote box|quote=It's the first time that we've observed a cataclysmic astrophysical event in both gravitational waves and electromagnetic waves—our cosmic messengers.{{cite web |url=https://news.mit.edu/2017/ligo-virgo-first-detection-gravitational-waves-colliding-neutron-stars-1016 |title=LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars |website=MIT News |date=16 October 2017 |access-date=23 October 2017}} | author = Reitze D |source=LIGO executive director |width=30%}}

The observations were officially announced on 16 October 2017 at press conferences at the National Press Club in Washington, D.C., and at the ESO headquarters in Garching bei München in Germany.{{cite news | vauthors = Overbye D |author-link=Dennis Overbye |title=LIGO detects fierce collision of neutron stars for the first time |url=https://www.nytimes.com/2017/10/16/science/ligo-neutron-stars-collision.html |date=16 October 2017 |newspaper=The New York Times |access-date=16 October 2017 }}{{cite news | vauthors = Krieger LM |title=A bright light seen across the Universe, proving Einstein right – violent collisions source of our gold, silver |url=http://www.mercurynews.com/2017/10/16/a-bright-light-seen-across-the-universe-proving-einstein-right/ |date=16 October 2017 |newspaper=The Mercury News |access-date=16 October 2017}}{{cite journal | vauthors = Cho A |title=Merging neutron stars generate gravitational waves and a celestial light show |date=16 October 2017 |journal=Science |doi=10.1126/science.aar2149 |url=https://www.science.org/content/article/merging-neutron-stars-generate-gravitational-waves-and-celestial-light-show}}

Some information was leaked before the official announcement, beginning on 18 August 2017 when astronomer J. Craig Wheeler of the University of Texas at Austin tweeted "New LIGO. Source with optical counterpart. Blow your sox off!"{{cite news | vauthors = Schilling G |title=Astronomers catch gravitational waves from colliding neutron stars |url=http://www.skyandtelescope.com/astronomy-news/astronomers-catch-gravitational-waves-from-colliding-neutron-stars/ |journal=Sky & Telescope |date=16 October 2017 |quote=because colliding black holes don't give off any light, you wouldn't expect any optical counterpart.}} He later deleted the tweet and apologized for scooping the official announcement embargo. Other people followed up on the rumor, and reported that the public logs of several major telescopes listed priority interruptions in order to observe {{nowrap|NGC 4993}}, a galaxy {{convert|40|Mpc|Mly|abbr=on|lk=on}} away in the Hydra constellation.{{cite news | vauthors = Castelvecchi D |title=Rumours swell over new kind of gravitational-wave sighting |date=August 2017 |journal=Nature News |doi=10.1038/nature.2017.22482}}{{cite news | vauthors = McKinnon M |title=Exclusive: We may have detected a new kind of gravitational wave |url=https://www.newscientist.com/article/2144937-exclusive-we-may-have-detected-a-new-kind-of-gravitational-wave/ |date=23 August 2017 |magazine=New Scientist |access-date=28 August 2017 }} The collaboration had earlier declined to comment on the rumors, not adding to a previous announcement that there were several triggers under analysis.{{cite news |title=A very exciting LIGO-Virgo observing run is drawing to a close August 25|url=http://www.ligo.org/news/index.php#O2end |date=25 August 2017 |publisher=LIGO |access-date=29 August 2017}}{{r|NG-20170825}}

File:Neutron star collision.ogv of the collision of two neutron stars. This is a general illustration, not specific to GW170817. (00:23 video.)]]

The gravitational wave signal lasted for approximately 100 seconds (much longer than the few seconds measured from binary black holes){{cite web |title=Variety of Gravitational Waves |url=https://www.ligo.caltech.edu/video/ligo20171016v3 |website=LIGO Lab Caltech}} starting from a frequency of 24 hertz. It covered approximately 3,000 cycles, increasing in amplitude and frequency to a few hundred hertz in the typical inspiral chirp pattern, ending with the collision received at 12:41:04.4 UTC.{{rp|2}} It arrived first at the Virgo detector in Italy, then 22 milliseconds later at the LIGO-Livingston detector in Louisiana, United States, and another 3 milliseconds later at the LIGO-Hanford detector in the state of Washington, in the United States. The signal was detected and analyzed by a comparison with a prediction from general relativity defined from the post-Newtonian expansion.{{rp|3}}

An automatic computer search of the LIGO-Hanford datastream triggered an alert to the LIGO team about 6 minutes after the event. The gamma-ray alert had already been issued at this point (16 seconds post-event),{{cite web |url=https://gcn.gsfc.nasa.gov/other/524666471.fermi |title=GCN notices related to Fermi-GBM alert 524666471 |date=17 August 2017 |access-date=19 October 2017 |department=Gamma-ray Burst Coordinates Network |publisher=NASA Goddard Space Flight Center}} so the timing near-coincidence was automatically flagged. The LIGO/Virgo team issued a preliminary alert (with only the crude gamma-ray position) to astronomers in the follow-up teams at 40 minutes post-event.{{r|GCN|Davide_16}}

Sky localisation of the event required combining data from the three interferometers, but this was delayed by two problems. The Virgo data were delayed by a data transmission problem, and the LIGO Livingston data were contaminated by a brief burst of instrumental noise a few seconds prior to the event peak, which persisted parallel to the rising transient signal in the lowest frequencies. These required manual analysis and interpolation before the sky location could be announced about 4.5 hours after the event.{{cite web |title=GW170817—The pot of gold at the end of the rainbow | vauthors = Christopher B |date=16 October 2017 |access-date=19 October 2017 |url=https://cplberry.com/2017/10/16/gw170817/ }}{{cite journal | vauthors = Castelvecchi D | title = Colliding stars spark rush to solve cosmic mysteries | journal = Nature | volume = 550 | issue = 7676 | pages = 309–310 | date = October 2017 | pmid = 29052641 | doi = 10.1038/550309a | doi-access = free | bibcode = 2017Natur.550..309C }} The three detections localized the source to an area of 31 square degrees in the southern sky at 90% probability. More detailed calculations later refined the localization to within 28 square degrees.{{r|GCN|PhysRev2017}} In particular, the absence of a clear detection by the Virgo interferometer implied that the source was localized within one of its blind spots, a constraint which reduced the search area considerably.{{cite journal | vauthors = Schilling GA |title=Two massive collisions and a Nobel Prize |journal=Sky & Telescope |volume=135 |issue=1 |page=10 |date=January 2018}}

File:Artist NSIllustration CREDIT NSF LIGO Sonoma State University A. Simonnet.jpg

The first electromagnetic signal detected was GRB 170817A, a short gamma-ray burst, detected {{val|1.74|0.05|u=seconds}} after the merger time and lasting for about 2 seconds.{{r|MN-20171016|NAT-20170825}}{{rp|5}}

GRB 170817A was first recorded by the Fermi Gamma-ray Space Telescope, which issued an automatic alert just 14 seconds after the detection. After the LIGO/Virgo circular 40 minutes later, manual processing of data from the INTEGRAL gamma-ray telescope retrieved independent data for the event. The difference in arrival time between Fermi and INTEGRAL helped to improve the sky localization.

This GRB was relatively faint given the proximity of the host galaxy {{nowrap|NGC 4993}}, possibly due to its jets not being pointed directly toward Earth, but rather at an angle of about 30 degrees off axis.{{r|SM-20171016}}{{cite web |url=https://www.space.com/38471-gravitational-waves-neutron-star-crashes-discovery-explained.html |title=Gravitational waves detected from neutron star crashes: The discovery explained | vauthors = Choi CQ |date=16 October 2017 |website=Space.com |publisher=Purch Group |access-date=16 October 2017}}

File:NGC 4993 and GRB170817A after glow.gif

File:Eso1733f.svg

File:Eso1733j X-shooter spectra montage of kilonova in NGC4993.png

A series of alerts to other astronomers were issued, beginning with a report of the gamma-ray detection and single-detector LIGO trigger at 13:21 UTC, and a three-detector sky location at 17:54 UTC.{{cite web |url=https://gcn.gsfc.nasa.gov/other/G298048.gcn3 |title=GCN circulars related to LIGO trigger G298048 |date=17 August 2017 |access-date=19 October 2017 |department=Gamma-ray Burst Coordinates Network |publisher=NASA Goddard Space Flight Center}} These prompted a massive search by many survey and robotic telescopes. In addition to the expected large size of the search area (about 150 times the area of a full moon), this search was challenging because the search area was near the Sun in the sky and thus visible for at most a few hours after dusk for any given telescope.{{r|Davide_16}}

In total six teams (One-Meter, Two Hemispheres (1M2H),Ryan Foley and Enrico Ramirez-Ruiz [https://ziggy.ucolick.org/sss17a/ (October 2017) GW170817/SSS17a: One-Meter, Two Hemispheres (1M2H)] DLT40, VISTA, Master, DECam, and Las Cumbres Observatory (Chile)) imaged the same new source independently in a 90-minute interval.{{r|ApJ|p=5}} The first to detect optical light associated with the collision was the 1M2H team running the Swope Supernova Survey, which found it in an image of {{nowrap|NGC 4993}} taken 10 hours and 52 minutes after the GW event{{r|MN-20171016|ApJ|Drout}} by the {{convert|1|m|ft|diameter|adj=mid|sp=us}} Swope Telescope operating in the near infrared at Las Campanas Observatory, Chile. They were also the first to announce it, naming their detection SSS17a in a circular issued 12{{sup|h}}26{{sup|m}} post-event. The new source was later given an official International Astronomical Union (IAU) designation AT 2017gfo.

The 1M2H team surveyed all galaxies in the region of space predicted by the gravitational wave observations, and identified a single new transient.{{r|spacecom|Drout}} By identifying the host galaxy of the merger, it is possible to provide an accurate distance consistent with that based on gravitational waves alone.{{r|ApJ|p=5}}

The detection of the optical and near-infrared source provided a huge improvement in localisation, reducing the uncertainty from several degrees to 0.0001 degree; this enabled many large ground and space telescopes to follow up the source over the following days and weeks.

Within hours after localization, many additional observations were made across the infrared and visible spectrum.{{r|Drout}} Over the following days, the color of the optical source changed from blue to red as the source expanded and cooled.{{r|spacecom}}

Numerous optical and infrared spectra were observed; early spectra were nearly featureless, but after a few days, broad features emerged indicative of material ejected at roughly 10 percent of light speed. There are multiple strong lines of evidence that AT 2017gfo is indeed the aftermath of GW170817. The color evolution and spectra are dramatically different from any known supernova. The distance of NGC 4993 is consistent with that independently estimated from the GW signal. No other transient has been found in the GW sky localisation region. Finally, various archive images show nothing at the location of AT 2017gfo, ruling out a foreground variable star in the Milky Way.

The source was detected in the ultraviolet (but not in X-rays) 15.3 hours after the event by the Swift Gamma-Ray Burst Mission.{{cite news |date=16 October 2017 |title=NASA missions catch first light from a gravitational-wave event |publisher=NASA |url=https://www.jpl.nasa.gov/news/news.php?feature=6975 |access-date=16 October 2017 |vauthors=Landau E, Chou F, Washington D, Porter M}} After initial lack of X-ray and radio detections, the source was detected in X-rays 9 days later{{Cite journal |last1=Troja |first1=E. |last2=Piro |first2=L. |last3=van Eerten |first3=H. |date=November 2017 |title=The X-ray counterpart to the gravitational-wave event GW170817 |url=https://www.nature.com/articles/nature24290 |journal=Nature |language=en |volume=551 |issue=7678 |pages=71–74 |doi=10.1038/nature24290 |issn=1476-4687|arxiv=1710.05433 |bibcode=2017Natur.551...71T |s2cid=205261229 }} using the Chandra X-ray Observatory,{{Cite web|url=http://chandra.si.edu/photo/2017/2nstars/|title=Chandra :: Photo Album :: GW170817 :: October 16, 2017|website=chandra.si.edu|access-date=16 August 2019}}{{Cite web|url=http://chandra.si.edu/blog/node/656|title=Chandra Makes First Detection of X-rays from a Gravitational Wave Source: Interview with Chandra Scientist Eleonora Nora Troja|website=chandra.si.edu|access-date=16 August 2019}} and 16 days later in the radio{{Cite journal |last1=Hallinan |first1=G. |last2=Corsi |first2=A. |title=A radio counterpart to a neutron star merger |journal=Science |year=2017 |volume=358 |issue=6370 |pages=1579–1583|doi=10.1126/science.aap9855 |pmid=29038372 |arxiv=1710.05435 |bibcode=2017Sci...358.1579H |s2cid=3974441 |url=https://authors.library.caltech.edu/81949/ }} using the Karl G. Jansky Very Large Array (VLA) in New Mexico.{{r|SM-20171016}} More than 70 observatories covering the electromagnetic spectrum observed the source.{{r|SM-20171016}}

The radio and X-ray light increased to a peak 150 days after the merger,{{cite journal|journal=Nature|title=A mildly relativistic wide-angle outflow in the neutron-star merger event GW170817|url=https://www.nature.com/articles/nature25452|arxiv=1711.11573|date=20 December 2017|doi=10.1038/nature25452 |last1=Mooley |first1=K. P. |last2=Nakar |first2=E. |last3=Hotokezaka |first3=K. |last4=Hallinan |first4=G. |last5=Corsi |first5=A. |last6=Frail |first6=D. A. |last7=Horesh |first7=A. |last8=Murphy |first8=T. |last9=Lenc |first9=E. |last10=Kaplan |first10=D. L. |last11=De |first11=K. |last12=Dobie |first12=D. |last13=Chandra |first13=P. |last14=Deller |first14=A. |last15=Gottlieb |first15=O. |last16=Kasliwal |first16=M. M. |last17=Kulkarni |first17=S. R. |last18=Myers |first18=S. T. |last19=Nissanke |first19=S. |last20=Piran |first20=T. |last21=Lynch |first21=C. |last22=Bhalerao |first22=V. |last23=Bourke |first23=S. |last24=Bannister |first24=K. W. |last25=Singer |first25=L. P. |volume=554 |issue=7691 |pages=207–210 |pmid=29261643 }}{{cite journal | arxiv=1803.06853 | doi=10.3847/2041-8213/aac105 | doi-access=free | title=A Turnover in the Radio Light Curve of GW170817 | date=2018 | last1=Dobie | first1=Dougal | last2=Kaplan | first2=David L. | last3=Murphy | first3=Tara | last4=Lenc | first4=Emil | last5=Mooley | first5=Kunal P. | last6=Lynch | first6=Christene | last7=Corsi | first7=Alessandra | last8=Frail | first8=Dale | last9=Kasliwal | first9=Mansi | last10=Hallinan | first10=Gregg | journal=The Astrophysical Journal Letters | volume=858 | issue=2 | pages=L15 | bibcode=2018ApJ...858L..15D }} diminishing afterwards.{{Cite web|url=http://theconversation.com/signals-from-a-spectacular-neutron-star-merger-that-made-gravitational-waves-are-slowly-fading-away-94294|title=Signals from a spectacular neutron star merger that made gravitational waves are slowly fading away| vauthors = Kaplan D, Murphy T |author-link2=Tara Murphy|website=The Conversation|date=30 April 2018 |access-date=16 August 2019}} Astronomers have monitored the optical afterglow of GW170817 using the Hubble Space Telescope.{{cite news | vauthors = Morris A |title=Hubble Captures Deepest Optical Image of First Neutron Star Collision |url=https://scitechdaily.com/hubble-captures-deepest-optical-image-of-first-neutron-star-collision/ |date=11 September 2019 |work=ScienceDaily.com |access-date=11 September 2019 }}{{Cite journal |last1=Lamb |first1=G. P. |last2=Lyman |first2=J. D. |last3=Levan |first3=A. J. |last4=Tanvir |first4=N. R. |last5=Kangas |first5=T. |last6=Fruchter |first6=A. S. |last7=Gompertz |first7=B. |last8=Hjorth |first8=J. |last9=Mandel |first9=I. |last10=Oates |first10=S. R. |last11=Steeghs |first11=D. |last12=Wiersema |first12=K. |date=2019-01-09 |title=The Optical Afterglow of GW170817 at One Year Post-merger |journal=The Astrophysical Journal |language=en |volume=870 |issue=2 |pages=L15 |doi=10.3847/2041-8213/aaf96b |arxiv=1811.11491 |bibcode=2019ApJ...870L..15L |issn=2041-8213|doi-access=free }} In March 2020, continued X-ray emission at 5-sigma was observed by the Chandra Observatory 940 days after the merger.{{cite news | vauthors = Troja E, Piro L, Ryan G, van Eerten H, Zhang B |title=ATel#13565 - GW170817: Continued X-ray emission detected with Chandra at 940 days post-merger |url=http://www.astronomerstelegram.org/?read=13565 |date=18 March 2020 |work=The Astronomer's Telegram |access-date=19 March 2020 }}

No neutrinos consistent with the source were found in follow-up searches by the IceCube and ANTARES neutrino observatories and the Pierre Auger Observatory.{{cite journal | vauthors = Abbott BP, Abbott R, Abbott TD, Acernese F, Ackley K, Adams C, Adams T, Addesso P, Adhikari RX, Adya VB, Affeldt C, Afrough M, Agarwal B, Agathos M, Agatsuma K, Aggarwal N, Aguiar OD, Aiello L, Ain A, Ajith P, Allen B, Allen G, Allocca A, Altin PA, Amato A, Ananyeva A, Anderson SB, Anderson WG, Angelova SV, Antier S, Appert S, Arai K, Araya MC, Areeda JS, Arnaud N, Arun KG, Ascenzi S, Ashton G, Ast M, Aston SM, Astone P, Atallah DV, Aufmuth P, Aulbert C, AultONeal K, Austin C, Avila-Alvarez A, Babak S, Bacon P, Bader MK, Bae S, Bailes M, Baker PT, Baldaccini F, Ballardin G, Ballmer SW, Banagiri S, Barayoga JC, Barclay SE, Barish BC, Barker D, Barkett K, Barone F, Barr B, Barsotti L, Barsuglia M, Barta D, Barthelmy SD, Bartlett J, Bartos I, Bassiri R, Basti A, Batch JC, Bawaj M, Bayley JC, Bazzan M, Bécsy B, Beer C, Bejger M, Belahcene I, Bell AS, Berger BK, Bergmann G, Bernuzzi S, Bero JJ, Berry CP, Bersanetti D, Bertolini A, Betzwieser J, Bhagwat S, Bhandare R, Bilenko IA, Billingsley G, Billman CR, Birch J, Birney R, Birnholtz O, Biscans S, Biscoveanu S, Bisht A, Bitossi M, Biwer C, Bizouard MA, Blackburn JK, Blackman J, Blair CD, Blair DG, Blair RM, Bloemen S, Bock O, Bode N, Boer M, Bogaert G, Bohe A, Bondu F, Bonilla E, Bonnand R, Boom BA, Bork R, Boschi V, Bose S, Bossie K, Bouffanais Y, Bozzi A, Bradaschia C, Brady PR, Branchesi M, Brau JE, Briant T, Brillet A, Brinkmann M, Brisson V, Brockill P, Broida JE, Brooks AF, Brown DA, Brown DD, Brunett S, Buchanan CC, Buikema A, Bulik T, Bulten HJ, Buonanno A, Buskulic D, Buy C, Byer RL, Cabero M, Cadonati L, Cagnoli G, Cahillane C, Calderón Bustillo J, Callister TA, Calloni E, Camp JB, Canepa M, Canizares P, Cannon KC, Cao H, Cao J, Capano CD, Capocasa E, Carbognani F, Caride S, Carney MF, Carullo G, Casanueva Diaz J, Casentini C, Caudill S, Cavaglià M, Cavalier F, Cavalieri R, Cella G, Cepeda CB, Cerdá-Durán P, Cerretani G, Cesarini E, Chamberlin SJ, Chan M, Chao S, Charlton P, Chase E, Chassande-Mottin E, Chatterjee D, Chatziioannou K, Cheeseboro BD, Chen HY, Chen X, Chen Y, Cheng HP, Chia H, Chincarini A, Chiummo A, Chmiel T, Cho HS, Cho M, Chow JH, Christensen N, Chu Q, Chua AJ, Chua S, Chung AK, Chung S, Ciani G, Ciolfi R, Cirelli CE, Cirone A, Clara F, Clark JA, Clearwater P, Cleva F, Cocchieri C, Coccia E, Cohadon PF, Cohen D, Colla A, Collette CG, Cominsky LR, Constancio M, Conti L, Cooper SJ, Corban P, Corbitt TR, Cordero-Carrión I, Corley KR, Cornish N, Corsi A, Cortese S, Costa CA, Coughlin MW, Coughlin SB, Coulon JP, Countryman ST, Couvares P, Covas PB, Cowan EE, Coward DM, Cowart MJ, Coyne DC, Coyne R, Creighton JD, Creighton TD, Cripe J, Crowder SG, Cullen TJ, Cumming A, Cunningham L, Cuoco E, Dal Canton T, Dálya G, Danilishin SL, D'Antonio S, Danzmann K, Dasgupta A, Da Silva Costa CF, Dattilo V, 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Thomas EG, Thomas M, Thomas P, Thorne KA, Thorne KS, Thrane E, Tiwari S, Tiwari V, Tokmakov KV, Toland K, Tonelli M, Tornasi Z, Torres-Forné A, Torrie CI, Töyrä D, Travasso F, Traylor G, Trinastic J, Tringali MC, Trozzo L, Tsang KW, Tse M, Tso R, Tsukada L, Tsuna D, Tuyenbayev D, Ueno K, Ugolini D, Unnikrishnan CS, Urban AL, Usman SA, Vahlbruch H, Vajente G, Valdes G, Vallisneri M, van Bakel N, van Beuzekom M, van den Brand JF, Van Den Broeck C, Vander-Hyde DC, van der Schaaf L, van Heijningen JV, van Veggel AA, Vardaro M, Varma V, Vass S, Vasúth M, Vecchio A, Vedovato G, Veitch J, Veitch PJ, Venkateswara K, Venugopalan G, Verkindt D, Vetrano F, Viceré A, Viets AD, Vinciguerra S, Vine DJ, Vinet JY, Vitale S, Vo T, Vocca H, Vorvick C, Vyatchanin SP, Wade AR, Wade LE, Wade M, Walet R, Walker M, Wallace L, Walsh S, Wang G, Wang H, Wang JZ, Wang WH, Wang YF, Ward RL, Warner J, Was M, Watchi J, Weaver B, Wei LW, Weinert M, Weinstein AJ, Weiss R, Wen L, Wessel EK, Weßels P, Westerweck J, Westphal T, Wette K, Whelan JT, Whitcomb SE, Whiting BF, Whittle C, Wilken D, Williams D, Williams RD, Williamson AR, Willis JL, Willke B, Wimmer MH, Winkler W, Wipf CC, Wittel H, Woan G, Woehler J, Wofford J, Wong KW, Worden J, Wright JL, Wu DS, Wysocki DM, Xiao S, Yamamoto H, Yancey CC, Yang L, Yap MJ, Yazback M, Yu H, Yu H, Yvert M, Zadrożny A, Zanolin M, Zelenova T, Zendri JP, Zevin M, Zhang L, Zhang M, Zhang T, Zhang YH, Zhao C, Zhou M, Zhou Z, Zhu SJ, Zhu XJ, Zimmerman AB, Zucker ME, Zweizig J | display-authors = 6 | title = GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral | journal = Physical Review Letters | volume = 119 | issue = 16 | pages = 161101 | date = October 2017 | pmid = 29099225 | doi = 10.1103/PhysRevLett.119.161101 | collaboration = LIGO Scientific Collaboration & Virgo Collaboration | arxiv = 1710.05832 | bibcode = 2017PhRvL.119p1101A | doi-access = free }}{{cite journal | vauthors = Abbott BP | display-authors = etal | collaboration = LIGO, Virgo and other collaborations | title = Multi-messenger Observations of a Binary Neutron Star Merger | journal = The Astrophysical Journal | date = October 2017 | volume = 848 | issue = 2 | page = L12 | doi = 10.3847/2041-8213/aa91c9 | doi-access = free | arxiv = 1710.05833 | url = https://dcc.ligo.org/public/0145/P1700294/007/ApJL-MMAP-171017.pdf | quote = The optical and near-infrared spectra over these few days provided convincing arguments that this transient was unlike any other discovered in extensive optical wide-field surveys over the past decade. | bibcode = 2017ApJ...848L..12A }} A possible explanation for the non-detection of neutrinos is because the event was observed at a large off-axis angle and thus the outflow jet was not directed towards Earth.{{cite journal | vauthors = Albert A, André M, Anghinolfi M, Ardid M, Aubert JJ, Aublin J, etal |collaboration=Antares Collaboration, IceCube Collaboration, Pierre Auger Collaboration, LIGO Scientific Collaboration, & Virgo Collaboration |title=Search for high-energy neutrinos from binary neutron star merger GW170817 with ANTARES, IceCube, and the Pierre Auger Observatory |journal=The Astrophysical Journal |volume=850 |issue=2 |pages=L35 |date=October 2017 |arxiv=1710.05839 |bibcode=2017ApJ...850L..35A |doi=10.3847/2041-8213/aa9aed |s2cid=217180814 |doi-access=free }}{{cite web | vauthors = Bravo S |title=No neutrino emission from a binary neutron star merger |url=http://icecube.wisc.edu/news/view/539 |website=IceCube South Pole Neutrino Observatory |date=16 October 2016 |access-date=20 October 2017}}

The origin and properties (masses and spins) of a double neutron star system like GW170817 are the result of a long sequence of complex binary star interactions.{{cite journal | vauthors = Tauris TM, Kramer M, Freire PC, Wex N, Janka H, Langer N, Podsiadlowski P, Bozzo E, Chaty S, Kruckow MU, Heuvel EP, Antoniadis J, Breton RP, Champion DJ | display-authors = 6 | title = Formation of Double Neutron Star Systems | journal = The Astrophysical Journal | date = 13 September 2017 | volume = 846 | issue = 2 | page = 170 | eissn = 1538-4357 | doi = 10.3847/1538-4357/aa7e89 | pmid = | arxiv = 1706.09438 | bibcode = 2017ApJ...846..170T | s2cid = 119471204 | url = | doi-access = free }} The gravitational wave signal indicated that it was produced by the collision of two neutron stars{{r|NAT-20170825|NS-20170823}}{{cite magazine | vauthors = Drake N |author-link=Nadia Drake |title=Strange stars caught wrinkling spacetime? Get the facts. |url=http://news.nationalgeographic.com/2017/08/new-gravitational-waves-neutron-stars-ligo-space-science/ |archive-url=https://web.archive.org/web/20170827011832/http://news.nationalgeographic.com/2017/08/new-gravitational-waves-neutron-stars-ligo-space-science/ |url-status=dead |archive-date=27 August 2017 |date=25 August 2017 |magazine=National Geographic |access-date=27 August 2017 }}{{cite magazine | vauthors = Sokol J |title=What happens when two neutron stars collide? Scientific revolution |url=https://www.wired.com/story/what-happens-when-two-neutron-stars-collide-scientific-revolution/ |date=25 August 2017 |magazine=Wired |access-date=27 August 2017 }} with a total mass of {{val|2.82|0.47|0.09}} solar masses ({{solar mass}}).{{r|PhysRev2017}} If low spins are assumed, consistent with those observed in binary neutron stars expected to merge within (twice{{efn|Without a Bayesian prior, the expected periodicity of a time series based on a single observation converges towards two times the observing period.}}) the Hubble time, the total mass is {{val|2.74|0.04|0.01|u=solar mass}}.

The masses of the progenitor stars have greater uncertainty. The chirp mass, a directly observable parameter which may be roughly equated to the geometric mean of the prior masses, was measured at {{val|1.188|0.004|0.002|u=solar mass}}.{{r|Abbott^3_AJT}} The larger progenitor ({{math|m₁}}) has a 90% probability of being between {{val|1.36|and|2.26|u=solar mass}}, and the smaller ({{math|m₂}}) has a 90% probability of being between {{val|0.86|and|1.36|u=solar mass}}.{{r|Abbott^3_AJT}} Under the low spin assumption, the ranges are {{val|1.36|to|1.60|u=solar mass}} for {{math|m₁}} and {{val|1.17|to|1.36|u=solar mass}} for {{math|m₂}}, inside a 12 km radius.{{cite journal |last1=Abbott |first1=B. P. |display-authors=etal |title=GW170817: Measurements of Neutron Star Radii and Equation of State |journal=Physical Review Letters |date=15 October 2018 |volume=121 |issue=16 |pages=161101 |doi=10.1103/PhysRevLett.121.161101 |pmid=30387654 |arxiv=1805.11581 |bibcode=2018PhRvL.121p1101A |quote=constrain R1=11.9+1.4−1.4  km and R2=11.9+1.4−1.4  km at the 90% credible level|doi-access=free }}

A hypermassive neutron star was believed to have formed initially, as evidenced by the large amount of ejecta (much of which would have been swallowed by an immediately forming black hole). At first, the lack of evidence for emissions being powered by neutron star spindown, which would occur for longer-surviving neutron stars, suggested it collapsed into a black hole within milliseconds.{{cite journal | vauthors = Margalit B, Metzger BD |title=Constraining the Maximum Mass of Neutron Stars from Multi-messenger Observations of GW170817 |journal=The Astrophysical Journal Letters |volume=850 |issue=2 |date=21 November 2017 |page=L19 |doi=10.3847/2041-8213/aa991c |doi-access=free |arxiv=1710.05938 |bibcode=2017ApJ...850L..19M }} However, a more detailed analysis of the GW170817 signal tail later found evidence of further features consistent with the seconds-long spindown of an intermediate or remnant hypermassive magnetar,{{cite journal | vauthors = van Putten MH, Della Valle M |title=Observational evidence for extended emission to GW170817 |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=482 |issue=1 |date=January 2019 |pages=L46–L49 |doi=10.1093/mnrasl/sly166 |doi-access=free |arxiv=1806.02165 |bibcode=2019MNRAS.482L..46V |quote=We report on a possible detection of extended emission (EE) in gravitational radiation during GRB170817A: a descending chirp with characteristic time-scale τ_s = {{val|3.01|0.2|u=s}} in a (H1,L1)-spectrogram up to 700 Hz with Gaussian equivalent level of confidence greater than 3.3 σ based on causality alone following edge detection applied to (H1,L1)-spectrograms merged by frequency coincidences.}} and the energy of this spindown was estimated at ≃63 Foe, equivalent to 3.5% of the mass-energy of our Sun.{{cite journal |last1=van Putten |first1=Maurice H. P. M. |title=The Central Engine of GRB170817A and the Energy Budget Issue: Kerr Black Hole versus Neutron Star in a Multi-Messenger Analysis |journal=Universe |page=5 |doi=10.3390/universe9060279 |url=https://www.researchgate.net/publication/371466362 |date=8 June 2023 |volume=9 |issue=6 |doi-access=free |bibcode=2023Univ....9..279V |quote=direct detection of a descending chirp in gravitational radiation during GRB170817A. Starting between the time of merger and the onset of GRB170817A, its total energy output Egw = (3.5 ±1)%Mc2 ≈ 6.3 ×10↑52 erg is emitted at gravitational-wave frequencies fgw ≤700 Hz}} This was below the estimated sensitivity of the LIGO search algorithms at the time.{{cite journal |author1 = The LIGO Scientific Collaboration|author2= the Virgo Collaboration|title=Search for gravitational waves from a long-lived remnant of the binary neutron star merger GW170817 |journal=The Astrophysical Journal |volume=875 |issue=2 |date=2018 |pages=160 |doi=10.3847/1538-4357/ab0f3d |doi-access=free |arxiv=1810.02581 |bibcode= |quote= }} This was confirmed in 2023 by a statistically independent method of analysis revealing the central engine of GRB{{nbsp}}170817A.{{cite journal |author1 = van Putten, M.H.P.M. |author2= Della Valle, M.|title=Central engine of GRB170817A: Neutron star versus Kerr black hole based on multimessenger calorimetry and event timing |journal=Astronomy & Astrophysics |volume=669 |date=2023 |pages=A36 |doi=10.1051/0004-6361/202142974 |doi-access=free |arxiv=2212.03295 |bibcode= 2023A&A...669A..36V|quote= }}

The short gamma-ray burst was followed over the next several months by its slower-evolving kilonova counterpart, a spherically expanding optical afterglow powered by the radioactive decay of heavy r-process nuclei produced and ejected at the initial cataclysmic instant.{{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}} GW170817 therefore confirmed neutron star mergers to be viable sites for the r-process, where the nucleosynthesis of around half the isotopes in elements heavier than iron can occur.{{r|SM-20171016}} A total of 16,000 times the mass of the Earth in heavy elements is believed to have formed, including approximately 10 Earth masses just of the two elements gold and platinum.{{cite AV media | vauthors = Berger E |time=1{{sup|h}}48{{sup|m}} |url=https://www.youtube.com/watch?v=mtLPKYl4AHs |title=LIGO/Virgo Press Conference |date=16 October 2017 |access-date=29 October 2017}} The electromagnetic emission is estimated at 0.5% of the mass-energy of our Sun.

{{As of|2025}}, the precise nature of the ultimately stable remnant remains uncertain.

File:Artist’s impression of strontium emerging from a neutron star merger.jpg

Scientific interest in the event was enormous, with dozens of preliminary papers (and almost 100 preprints{{cite web |title=ArXiv.org search for GW170817 |url=https://arxiv.org/find/all/1/all:+GW170817/0/1/0/all/0/1 |access-date=18 October 2017}}) published the day of the announcement, including 8 letters in Science,{{r|SM-20171016}} 6 in Nature, and 32 in a special issue of The Astrophysical Journal Letters devoted to the subject.{{cite journal |title=Focus on the electromagnetic counterpart of the neutron star binary merger GW170817 |journal=The Astrophysical Journal Letters |volume=848 |issue=2 | vauthors = Berger E |type=Editorial |url=https://iopscience.iop.org/journal/2041-8205/page/Focus_on_GW170817 |date=16 October 2017 |quote=It is rare for the birth of a new field of astrophysics to be pinpointed to a singular event. This focus issue follows such an event – the neutron star binary merger GW170817 – marking the first joint detection and study of gravitational waves (GWs) and electromagnetic radiation (EM).}} The interest and effort was global: The paper describing the multi-messenger observations{{r|ApJ}} is coauthored by almost 4,000 astronomers (about one-third of the worldwide astronomical community) from more than 900 institutions, using more than 70 observatories on all 7 continents and in space.{{r|SkyandTelescope}}

The event also provided a limit on the difference between the speed of light and that of gravity. Assuming the first photons were emitted between zero and ten seconds after peak gravitational wave emission, the difference between the speeds of gravitational and electromagnetic waves, v_GW − v_EM, is constrained to between −3×10⁻¹⁵ and +7×10⁻¹⁶ times the speed of light, which improves on the previous estimate by about 14 orders of magnitude.{{Cite journal |doi=10.3847/2041-8213/aa920c |doi-access=free |volume=848 |issue=2 |page=L13 | vauthors = Abbott BP, Abbott R, Abbott TD, Acernese F, Ackley K, Adams C, Adams T, Addesso P, Adhikari RX, Adya VB, Affeldt C | display-authors = 6 |title=Gravitational waves and gamma-rays from a binary neutron star merger: GW170817 and GRB 170817A |journal=The Astrophysical Journal Letters |date=2017 |arxiv=1710.05834 |bibcode=2017ApJ...848L..13A}}{{cite journal |title=Viewpoint: Reining in Alternative Gravity | vauthors = Schmidt F |date=18 December 2017 |journal=Physics |volume=10 | page = 134 |doi=10.1103/physics.10.134|doi-access=free }}{{efn|The previous constraint on the difference between the speeds of light and gravity was about ±20%.}}

In addition, GW170817 allowed investigation of the equivalence principle (through Shapiro delay measurement) and Lorentz invariance.{{r|PhysRev2017}} The limits of possible violations of Lorentz invariance (values of 'gravity sector coefficients') are reduced by the new observations by up to ten orders of magnitude.{{r|Abbott^3_AJT}}

The event also excluded some alternatives to general relativity,{{cite news |title=How crashing neutron stars killed off some of our best ideas about what 'dark energy' is |date=13 December 2017 | vauthors = Kitching T |journal=The Conversation |via =phys.org |url=https://phys.org/news/2017-12-neutron-stars-ideas-dark-energy.html }} including variants of scalar–tensor theory,{{cite journal |title=Breaking a Dark Degeneracy with Gravitational Waves |journal=Journal of Cosmology and Astroparticle Physics |volume=2016 |issue=3 |pages=031 | vauthors = Lombriser L, Taylor A |arxiv=1509.08458 |date=28 September 2015 |doi=10.1088/1475-7516/2016/03/031 |bibcode=2016JCAP...03..031L|s2cid=73517974 }}{{cite journal |title=Challenges to self-acceleration in modified gravity from gravitational waves and large-scale structure |journal=Phys. Lett. B |volume=765 |pages=382–385 | vauthors = Lombriser L, Lima N |arxiv=1602.07670 |year=2017 |doi=10.1016/j.physletb.2016.12.048 |bibcode=2017PhLB..765..382L|s2cid=118486016 }}{{Cite journal | vauthors = Bettoni D, Ezquiaga JM, Hinterbichler K, Zumalacárregui M |first4=Miguel |date=14 April 2017 |title=Speed of gravitational waves and the fate of Scalar-Tensor Gravity |arxiv=1608.01982 |journal=Physical Review D |volume=95 |issue=8 |pages=084029 |doi=10.1103/PhysRevD.95.084029 |issn=2470-0010 |bibcode=2017PhRvD..95h4029B|s2cid=119186001 }}{{cite journal | vauthors = Creminelli P, Vernizzi F | title = Dark Energy after GW170817 and GRB170817A | journal = Physical Review Letters | volume = 119 | issue = 25 | pages = 251302 | date = December 2017 | pmid = 29303308 | doi = 10.1103/PhysRevLett.119.251302 | arxiv = 1710.05877 | s2cid = 206304918 | bibcode = 2017PhRvL.119y1302C }}{{cite journal | vauthors = Ezquiaga JM, Zumalacárregui M | title = Dark Energy After GW170817: Dead Ends and the Road Ahead | journal = Physical Review Letters | volume = 119 | issue = 25 | pages = 251304 | date = December 2017 | pmid = 29303304 | doi = 10.1103/PhysRevLett.119.251304 | arxiv = 1710.05901 | s2cid = 38618360 | bibcode = 2017PhRvL.119y1304E }}{{cite news |url=https://phys.org/news/2017-02-quest-riddle-einstein-theory.html |title=Quest to settle riddle over Einstein's theory may soon be over |date=10 February 2017 |access-date=29 October 2017 |website=phys.org}}{{cite news |url=https://arstechnica.co.uk/science/2017/02/theoretical-battle-dark-energy-vs-modified-gravity/ |title=Theoretical battle: Dark energy vs. modified gravity |date=25 February 2017 |access-date=27 October 2017 |website=Ars Technica}}{{Cite news |url=https://www.sciencenews.org/editors-picks/gravitational-waves |title=Gravitational waves |newspaper=Science News |access-date=1 November 2017 }} Hořava–Lifshitz gravity,{{cite journal | vauthors = Sakstein J, Jain B | title = Implications of the Neutron Star Merger GW170817 for Cosmological Scalar-Tensor Theories | journal = Physical Review Letters | volume = 119 | issue = 25 | pages = 251303 | date = December 2017 | pmid = 29303345 | doi = 10.1103/PhysRevLett.119.251303 | arxiv = 1710.05893 | s2cid = 39068360 | bibcode = 2017PhRvL.119y1303S }} Dark Matter Emulators,{{Cite journal | vauthors = Boran S, Desai S, Kahya E, Woodard R |year=2018 |title=GW170817 falsifies dark matter emulators |journal=Phys. Rev. D |volume=97 |issue=4 |pages=041501 |arxiv=1710.06168 |doi=10.1103/PhysRevD.97.041501 |bibcode=2018PhRvD..97d1501B|s2cid=119468128}} and bimetric gravity,{{cite journal | vauthors = Baker T, Bellini E, Ferreira PG, Lagos M, Noller J, Sawicki I | title = Strong Constraints on Cosmological Gravity from GW170817 and GRB 170817A | journal = Physical Review Letters | volume = 119 | issue = 25 | pages = 251301 | date = December 2017 | pmid = 29303333 | doi = 10.1103/PhysRevLett.119.251301 | arxiv = 1710.06394 | s2cid = 36160359 | bibcode = 2017PhRvL.119y1301B }} Furthermore, an analysis published in July 2018 used GW170817 to show that gravitational waves propagate fully through the 3+1 curved spacetime described by general relativity, ruling out hypotheses involving "leakage" into higher, non-compact spatial dimensions.{{efn|Compactified dimensions cannot be ruled out by GW studies because the fundamental diffraction limit for waves with frequencies in the tens to hundreds of Hz limits their probative resolution to scales no smaller than thousands of kilometers, leaving features below this scale unresolved.}}{{cite journal |last1=Pardo |first1=Kris |last2=Fishbach |first2=Maya |last3=Holz |first3=Daniel E. |last4=Spergel |first4=David N. |year=2018 |title=Limits on the number of spacetime dimensions from GW170817 |journal=Journal of Cosmology and Astroparticle Physics |volume=2018 |issue=7 |page=048 |arxiv=1801.08160 |bibcode=2018JCAP...07..048P |doi=10.1088/1475-7516/2018/07/048 |s2cid=119197181}}

Gravitational wave signals such as GW170817 may be used as a standard siren to provide an independent measurement of the Hubble constant.{{cite journal | title = A gravitational-wave standard siren measurement of the Hubble constant | journal = Nature | volume = 551 | issue = 7678 | pages = 85–88 | date = November 2017 | pmid = 29094696 | doi = 10.1038/nature24471 | arxiv = 1710.05835 | s2cid = 205261622 | bibcode = 2017Natur.551...85A | vauthors = Abbott BP, Abbott R, Abbott TD, Acernese F, Ackley K, Adams C, Adams T, Addesso P, Adhikari RX, Adya VB, Affeldt C, Afrough M, Agarwal B, Agathos M, Agatsuma K, Aggarwal N, Aguiar OD, Aiello L, Ain A, Ajith P, Allen B, Allen G, Allocca A, Altin PA, Amato A, Ananyeva A, Anderson SB, Anderson WG, Angelova SV, Antier S | display-authors = 6 }}{{cite magazine | vauthors = Scharping N |title=Gravitational waves show how fast the Universe is expanding |url=http://www.astronomy.com/news/2017/10/gravitational-waves-show-how-fast-the-universe-is-expanding |date=18 October 2017 |magazine=Astronomy |access-date=18 October 2017}} An initial estimate of the constant derived from the observation is {{val|70.0|+12.0|-8.0}} (km/s)/Mpc, broadly consistent with current best estimates.{{r|Nat24471}} Further studies improved the measurement to {{val|70.3|+5.3|-5.0}} (km/s)/Mpc.{{cite journal | vauthors = Hotokezaka K, Nakar E, Gottlieb O, Nissanke S, Masuda K, Hallinan G, Mooley KP, Deller AT | display-authors = 6 |title=A Hubble constant measurement from superluminal motion of the jet in GW170817 |url=https://www.nature.com/articles/s41550-019-0820-1 |date=8 July 2019 |journal=Nature Astronomy |volume=3 |issue=10 |pages=940–944 |doi=10.1038/s41550-019-0820-1 |access-date=8 July 2019 |bibcode=2019NatAs...3..940H |arxiv=1806.10596 | hdl = 2066/208868 |s2cid=119547153 }}{{cite news |publisher=National Radio Astronomy Observatory |title=New method may resolve difficulty in measuring universe's expansion – Neutron star mergers can provide new 'cosmic ruler' |url=https://www.eurekalert.org/pub_releases/2019-07/nrao-nmm070819.php |date=8 July 2019 |via=EurekAlert! |access-date=8 July 2019}}{{cite news | vauthors = Finley D |title=New method may resolve difficulty in measuring Universe's expansion |url=https://public.nrao.edu/news/new-method-measuring-universe-expansion/ |date=8 July 2019 |publisher=National Radio Astronomy Observatory |access-date=8 July 2019 }} Together with the observation of future events of this kind, the uncertainty is expected to reach two percent within five years and one percent within ten years.{{cite web | vauthors = Lerner L |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 |via=Phys.org |access-date=22 October 2018 }}{{cite journal | vauthors = Chen HY, Fishbach M, Holz DE | title = A two per cent Hubble constant measurement from standard sirens within five years | journal = Nature | volume = 562 | issue = 7728 | pages = 545–547 | date = October 2018 | pmid = 30333628 | doi = 10.1038/s41586-018-0606-0 | arxiv = 1712.06531 | s2cid = 52987203 | bibcode = 2018Natur.562..545C }}

Electromagnetic observations help support the theory that neutron star mergers contribute to rapid neutron capture (r-process) nucleosynthesis{{cite journal | vauthors = Drout MR, Piro AL, Shappee BJ, Kilpatrick CD, Simon JD, Contreras C, Coulter DA, Foley RJ, Siebert MR, Morrell N, Boutsia K, Di Mille F, Holoien TW, Kasen D, Kollmeier JA, Madore BF, Monson AJ, Murguia-Berthier A, Pan YC, Prochaska JX, Ramirez-Ruiz E, Rest A, Adams C, Alatalo K, Bañados E, Baughman J, Beers TC, Bernstein RA, Bitsakis T, Campillay A, Hansen TT, Higgs CR, Ji AP, Maravelias G, Marshall JL, Bidin CM, Prieto JL, Rasmussen KC, Rojas-Bravo C, Strom AL, Ulloa N, Vargas-González J, Wan Z, Whitten DD | display-authors = 6 | title = Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesis | journal = Science | volume = 358 | issue = 6370 | pages = 1570–1574 | date = December 2017 | pmid = 29038375 | doi = 10.1126/science.aaq0049 | arxiv = 1710.05443 | bibcode = 2017Sci...358.1570D | doi-access = free }}—previously assumed to be associated with supernova explosions—and are therefore the primary source of r-process elements heavier than iron,{{r|ApJ}} including gold and platinum.{{r|EdoBerger}} The first identification of r-process elements in a neutron star merger was obtained during a re-analysis of GW170817 spectra.{{cite journal |author8-link=Almudena Arcones |display-authors=6 |vauthors=Watson D, Hansen CJ, Selsing J, Koch A, Malesani DB, Andersen AC, Fynbo JP, Arcones A, Bauswein A, Covino S, Grado A, Heintz KE, Hunt L, Kouveliotou C, Leloudas G, Levan AJ, Mazzali P, Pian E |date=October 2019 |title=Identification of strontium in the merger of two neutron stars |journal=Nature |volume=574 |issue=7779 |pages=497–500 |arxiv=1910.10510 |bibcode=2019Natur.574..497W |doi=10.1038/s41586-019-1676-3 |pmid=31645733 |s2cid=204837882}} The spectra provided direct proof of strontium production during a neutron star merger. This also provided the most direct proof that neutron stars are made of neutron-rich matter. Since then, several r-process elements have been identified in the ejecta including yttrium,{{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 }} lanthanum and cerium.{{Cite journal |last1=Domoto |first1=Nanae |last2=Tanaka |first2=Masaomi |last3=Kato |first3=Daiji |last4=Kawaguchi |first4=Kyohei |last5=Hotokezaka |first5=Kenta |last6=Wanajo |first6=Shinya |date=2022-10-26 |title=Lanthanide Features in Near-infrared Spectra of Kilonovae |journal=The Astrophysical Journal |volume=939 |issue=1 |pages=8 |doi=10.3847/1538-4357/ac8c36 |doi-access=free |issn=0004-637X|arxiv=2206.04232 |bibcode=2022ApJ...939....8D }}

In October 2017, Stephen Hawking, in his last broadcast interview, discussed the overall scientific importance of GW170817.{{cite news |date=26 March 2018 |title=Stephen Hawking's final interview: A beautiful Universe |work=BBC News |url=https://www.bbc.com/news/science-environment-43499024 |access-date=26 March 2018 |vauthors=Ghosh P}} In September 2018, astronomers reported related studies about possible mergers of neutron stars (NS) and white dwarfs (WD): including NS-NS, NS-WD, and WD-WD mergers.{{cite journal |display-authors=6 |vauthors=Rueda JA, Ruffini R, Wang Y, Aimuratov Y, de Almeida UB, Bianco CL, Chen YC, Lobato RV, Maia C, Primorac D, Moradi R |date=28 September 2018 |title=GRB 170817A-GW170817-AT 2017gfo and the observations of NS-NS, NS-WD, and WD-WD mergers |journal=Journal of Cosmology and Astroparticle Physics |volume=2018 |issue=10 |page=006 |arxiv=1802.10027 |bibcode=2018JCAP...10..006R |doi=10.1088/1475-7516/2018/10/006 |s2cid=119369873}}

In October 2018, astronomers reported that, in retrospect, an sGRB event detected in 2015 ({{nowrap|GRB 150101B}}) may represent an earlier case of the same astrophysics reported for 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, were considered "striking", suggesting that the earlier event may also be the result of a neutron star merger, and that together these may signify a hitherto-unknown class of kilonova transients, making kilonovae more diverse and common in the universe than previously understood.{{cite news |publisher=University of Maryland |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. |url=https://www.eurekalert.org/pub_releases/2018-10/uom-ait101518.php |date=16 October 2018 |via=EurekAlert! |access-date=17 October 2018}}{{cite journal | vauthors = Troja E, Ryan G, Piro L, van Eerten H, Cenko SB, Yoon Y, Lee SK, Im M, Sakamoto T, Gatkine P, Kutyrev A, Veilleux S | display-authors = 6 | 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 | date = October 2018 | pmid = 30327476 | pmc = 6191439 | doi = 10.1038/s41467-018-06558-7 | arxiv = 1806.10624 | bibcode = 2018NatCo...9.4089T }}{{cite news | vauthors = Mohon L |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 |publisher=NASA |access-date=17 October 2018 }}{{cite web | vauthors = Wall M |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 |website=Space.com |access-date=17 October 2018}}

Later research further construed {{nowrap|GRB 160821B}}—another sGRB predating GW170817—also to belong to this class, again based on afterglow resemblance to the {{nowrap|AT 2017gfo}} signature.{{Cite journal |doi = 10.1093/mnras/stz2255|arxiv = 1905.01290|bibcode = 2019MNRAS.489.2104T|title = The afterglow and kilonova of the short GRB 160821B|year = 2019| vauthors = Troja E, Castro-Tirado AJ, Becerra González J, Hu Y, Ryan GS, Cenko SB, Ricci R, Novara G, Sánchez-Rámirez R, Acosta-Pulido JA, Ackley KD, Caballero García MD, Eikenberry SS, Guziy S, Jeong S, Lien AY, Márquez I, Pandey SB, Park IH, Sakamoto T, Tello JC, Sokolov IV, Sokolov VV, Tiengo A, Valeev AF, Zhang BB, Veilleux S | display-authors = 6 |journal = Monthly Notices of the Royal Astronomical Society|volume = 489|issue = 2|pages = 2104| doi-access=free |s2cid = 145047934}}

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• {{cite web |url=http://ligo.org/detections.php |title=Detections |publisher=LIGO}}
• {{cite web |url=https://kilonova.space/kne/GW170817/ |title=Follow-up observations of GW170817 |access-date=20 October 2017 |archive-date=17 September 2018 |archive-url=https://web.archive.org/web/20180917143430/https://kilonova.space/kne/GW170817/ |url-status=dead }}
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• {{youTube|mtLPKYl4AHs|NSF LIGO-Virgo press conference: 2 panels and Q&As (03:21)}}
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