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'''GW170817''' is a [[gravitational wave]] (GW) signal observed by the [[LIGO]] and [[Virgo interferometer|Virgo]] detectors on 17 August 2017. The GW was produced by the last minutes of two [[neutron star]]s [[inspiral|spiralling closer]] to each other and finally [[Neutron star merger|merging]], and is the first time a GW observation has been confirmed by non-gravitational means.
'''GW170817''' is a [[gravitational wave]] (GW) signal observed by the [[LIGO]] and [[Virgo interferometer|Virgo]] detectors on 17 August 2017. The GW was produced by the last minutes of two [[neutron star]]s [[inspiral|spiralling closer]] to each other and finally [[Neutron star merger|merging]], and is the first time a GW observation has been confirmed by non-gravitational means.


Unlike all previous GW detections, which were of merging [[black holes]] not expected to produce a detectable [[electromagnetic signal]],<ref>{{cite journal
Unlike all previous GW detections, which were of merging [[black holes]] not expected to produce a detectable [[electromagnetic signal]],<ref>{{cite journal|last=Connaughton|first=Valerie|date=|year=2016|title=Focus on Electromagnetic Counterparts to Binary Black Hole Mergers|url=http://iopscience.iop.org/journal/2041-8205/page/Focus_on_BBHM|journal=[[The Astrophysical Journal Letters]]|type=Editorial|volume=|issue=|pages=|quote=The follow-up observers sprang into action, not expecting to detect a signal if the gravitational radiation was indeed from a binary black-hole merger. [...] most observers and theorists agreed: the presence of at least one neutron star in the binary system was a prerequisite for the production of a circumbinary disk or neutron star ejecta, without which no electromagnetic counterpart was expected.|via=}}</ref><ref>{{Cite web|url=http://iopscience.iop.org/journal/2041-8205/page/Focus_on_GWS|title=Focus on the Search for Electromagnetic Counterparts to Gravitational-wave Sources - The Astrophysical Journal Letters - IOPscience|website=iopscience.iop.org|language=en|access-date=2017-10-29}}</ref>{{efn
|title=Focus on Electromagnetic Counterparts to Binary Black Hole Mergers
⚫
|Although acknowledged as unlikely, several mechanisms have been suggested by which a black hole merger could be surrounded by sufficient matter to produce an electromagnetic signal, which astronomers have been searching for.
|first=Valerie |last=Connaughton
⚫
}} the aftermath of this merger was also seen by many conventional [[telescope]]s, as well as [[gamma-ray astronomy|gamma-ray detection satellites]], marking a significant breakthrough for [[multi-messenger astronomy]].<ref>{{Cite web|url=https://www.ligo.caltech.edu/page/press-release-gw170817|title=GW170817 Press Release|website=LIGO Lab {{!}} Caltech|access-date=2017-10-29}}</ref><ref name="NASA-20171016">{{cite news
|type=Introduction
|journal=[[The Astrophysical Journal Letters]]
|year=2016
|volume=826
|issue=1
|url=http://iopscience.iop.org/journal/2041-8205/page/Focus_on_BBHM
|quote=The follow-up observers sprang into action, not expecting to detect a signal if the gravitational radiation was indeed from a binary black-hole merger. [...] most observers and theorists agreed: the presence of at least one neutron star in the binary system was a prerequisite for the production of a circumbinary disk or neutron star ejecta, without which no electromagnetic counterpart was expected.
}}</ref><ref name=Loeb>{{cite journal
|title=Electromagnetic Counterparts to Black Hole Mergers Detected by LIGO
|last=Loeb |first=Abraham
|journal=[[The Astrophysical Journal Letters]] |volume=819 |issue=2 |page=L21 |date=March 2016
|arxiv=1602.04735 |class=astro-ph.HE
|doi=10.3847/2041-8205/819/2/L21 |doi-access=free |bibcode=2016ApJ...819L..21L
|url=http://iopscience.iop.org/article/10.3847/2041-8205/819/2/L21/pdf
|quote=Mergers of stellar-mass black holes (BHs) [...] are not expected to have electromagnetic counterparts. [...] I show that the [GW and gamma-ray] signals might be related if the BH binary detected by LIGO originated from two clumps in a dumbbell configuration that formed when the core of a rapidly rotating massive star collapsed.
}}</ref>{{efn
⚫
|Although acknowledged as unlikely, several mechanisms have been suggested by which a black hole merger could be surrounded by sufficient matter to produce an electromagnetic signal, which astronomers have been searching for.{{r|Loeb}}<ref>{{cite journal
|last1=de Mink |first1=S.E.
|last2=King |first2=A.
|title=Electromagnetic Signals Following Stellar-mass Black Hole Mergers
|journal=[[The Astrophysical Journal Letters]] |date=April 2017 |volume=839 |issue=1 |page=L7
|doi=10.3847/2041-8213/aa67f3
|arxiv=1703.07794 |class=astro-ph.HE
|bibcode=2017ApJ...839L...7D
|url=https://lra.le.ac.uk/bitstream/2381/39732/6/Mink_2017_ApJL_839_L7.pdf
|quote=It is often assumed that gravitational-wave (GW) events resulting from the merger of stellar-mass black holes are unlikely to produce electromagnetic (EM) counterparts. We point out that the progenitor binary has probably shed a mass {{solar mass|≳10}} during its prior evolution. If even a tiny fraction of this gas is retained in a circumbinary disk, the sudden mass loss and recoil of the merged black hole shocks and heats it within hours of the GW event. Whether the resulting EM signal is detectable is uncertain.
}}</ref>
⚫
}} the aftermath of this merger was also seen by many conventional [[telescope]]s, as well as [[gamma-ray astronomy|gamma-ray detection satellites]], marking a significant breakthrough for [[multi-messenger astronomy]].{{r|PhysRev2017|ApJL}}<ref name="NASA-20171016">{{cite news
|last1=Landau |first1=Elizabeth |last2=Chou |first2=Felicia
|last1=Landau |first1=Elizabeth |last2=Chou |first2=Felicia
|last3=Washington |first3=Dewayne |last4=Porter |first4=Molly
|last3=Washington |first3=Dewayne |last4=Porter |first4=Molly
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|url=https://www.jpl.nasa.gov/news/news.php?feature=6975
|url=https://www.jpl.nasa.gov/news/news.php?feature=6975
|date=16 October 2017 |publisher=[[NASA]] |accessdate=16 October 2017
|date=16 October 2017 |publisher=[[NASA]] |accessdate=16 October 2017
}}</ref><ref>{{cite news
}}</ref><ref>{{Cite web|url=https://www.eso.org/public/unitedkingdom/images/eso1733u/|title=GW170817: a global astronomy event|last=information@eso.org|website=www.eso.org|language=en-GB|access-date=2017-10-29}}</ref>
|url=https://www.csmonitor.com/Science/2017/1016/Neutron-star-discovery-marks-breakthrough-for-multi-messenger-astronomy
|title=Neutron star discovery marks breakthrough for 'multi-messenger astronomy'
|first=Eva |last=Botkin-Kowacki
|journal=[[The Christian Science Monitor]]
|date=16 October 2017 |accessdate=17 October 2017
}}</ref><ref>{{cite arxiv
|title=Welcome to the Multi-Messenger Era! Lessons from a Neutron Star Merger and the Landscape Ahead
|first=Brian D. |last=Metzger
|date=16 October 2017
|arxiv=1710.05931 |class=astro-ph.HE
}}</ref>


Technically, there were four separate observations, and strong evidence that they came from the same astronomical source:
Technically, there were three separate observations, and strong evidence that they came from the same astronomical source:
* The GW signal, which had a duration of approximately 100 seconds, shows the characteristics in intensity and frequency expected of the [[inspiral]] of two neutron stars. Analysis of the slight variation in arrival time of the GW at the three detector locations (two LIGO and one Virgo) yielded an approximate angular [[direction finding|direction to the source]].
* The GW signal, which had a duration of approximately 100 seconds, shows the characteristics in intensity and frequency expected of the [[inspiral]] of two neutron stars. Analysis of the slight variation in arrival time of the GW at the three detector locations (two LIGO and one Virgo) yielded an approximate angular [[direction finding|direction to the source]].
* The short [[gamma-ray burst]] [[GRB 170817A]]{{r|NYT-20171016|MN-20171016}} detected by the [[Fermi Gamma-ray Space Telescope|Fermi]] and [[INTEGRAL]] spacecraft 1.7&nbsp;seconds after the GW signal ended<ref name="NASA-20171016" />. These detectors have very limited directional sensitivity, but indicated a large area of the sky which overlapped the gravitational wave position. It has long been theorized that short gamma-ray bursts are caused by neutron star mergers.
* The short [[gamma-ray burst]] [[GRB 170817A]]{{r|NYT-20171016|MN-20171016}} detected by the [[Fermi Gamma-ray Space Telescope|Fermi]] and [[INTEGRAL]] spacecraft 1.7&nbsp;seconds after the GW signal ended. These detectors have very limited directional sensitivity, but indicated a large area of the sky which overlapped the gravitational wave position. It has long been theorized that short gamma-ray bursts are caused by neutron star mergers.
* The optical [[Transient astronomical event|transient]] [[AT 2017gfo]], found 11 hours later in the galaxy [[NGC 4993]]<ref name="SM-20171016">{{cite journal|last=Cho|first=Adrian|date=October 2017|title=Merging neutron stars generate gravitational waves and a celestial light show|journal=[[Science (magazine)|Science]]|doi=10.1126/science.aar2149}}</ref> during a search of the region indicated by the GW detection. This was observed by numerous telescopes at optical and infrared wavelengths over the following days and weeks, and shows the characteristics (a fast-moving, rapidly-cooling cloud of neutron-rich material) expected of debris ejected from a [[Neutron star|neutron-star]] merger.
* The optical [[Transient astronomical event|transient]] [[AT 2017gfo]], found 11 hours later in the galaxy [[NGC 4993]]<ref name="SM-20171016" /> during a search of the region indicated by the GW detection. This was observed by numerous telescopes, from radio to X-ray wavelengths, over the following days and weeks, and shows the characteristics (a fast-moving, rapidly-cooling cloud of neutron-rich material) expected of debris ejected from a [[Neutron star|neutron-star]] merger.
* Subsequent observations at [[X-ray]]<ref name="NASA-20171016" /><ref name=":0">{{Cite news|url=https://www.nasa.gov/mission_pages/chandra/images/chandra-makes-first-detection-of-x-rays-from-a-gravitational-wave-source.html|title=Chandra Makes First Detection of X-Rays from Gravitational Wave Source|last=Mohon|first=Lee|date=2017-10-16|work=NASA|access-date=2017-10-29|language=en}}</ref><ref name="APJ" /> and radio<ref name="APJ" /><ref>{{Cite web|url=https://public.nrao.edu/news/radio-eyes-unlocking-secrets/|title=Radio "Eyes" Unlocking Secrets of Neutron-Star Collision - National Radio Astronomy Observatory|website=public.nrao.edu|language=en-US|access-date=2017-10-29}}</ref> wavelengths revealed the [[Gamma-ray burst|GRB afterglow]] from the short duration [[GRB 170817A]] at the same location of the transient [[Transient astronomical event|transient]] [[AT 2017gfo]].


==Announcement==
==Announcement==
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|width = 30%
|width = 30%
}}
}}
The event was officially announced on 16 October 2017<ref name="NYT-20171016">{{cite news |last=Overbye |first=Dennis |authorlink=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 |work=[[The New York Times]] |accessdate=16 October 2017 }}</ref><ref name="MN-20171016">{{cite news |last=Krieger |first=Lisa M. |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 |work=[[The Mercury News]] |accessdate=16 October 2017 }}</ref> at press conferences at the [[National Press Club (United States)|National Press Club]] in [[Washington, D.C.]] and at the [[European Southern Observatory|ESO]] headquarters in [[Garching bei München]] in Germany.<ref name="SM-20171016">{{cite journal |last=Cho |first=Adrian |title=Merging neutron stars generate gravitational waves and a celestial light show |date=October 2017 |journal=[[Science (magazine)|Science]] |doi=10.1126/science.aar2149}}</ref>
The event was officially announced on 16 October 2017 at press conferences at the [[National Press Club (United States)|National Press Club]] in [[Washington, D.C.]]<ref>{{Cite web|url=https://nsf.gov/news/news_summ.jsp?cntn_id=243382&org=NSF&from=news|title=LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars {{!}} NSF - National Science Foundation|website=nsf.gov|language=en|access-date=2017-10-29}}</ref> and at the [[European Southern Observatory|ESO]] headquarters<ref>{{Cite web|url=http://www.eso.org/public/usa/announcements/ann16057/|title=Media Advisory: Press Conference at ESO HQ to Make Important Announcement|last=information@eso.org|website=www.eso.org|language=en-US|access-date=2017-10-29}}</ref> in [[Garching bei München]] in Germany.


The first public information about the event was tweeted by astronomer J. Craig Wheeler of the [[University of Texas at Austin]] on 18 August 2017. He later deleted the tweet and apologized for [[Scoop (news)|scooping]] the official announcement protocol. Other people followed up on the rumor, and reported that the public logs of several major telescopes listed priority interruptions in order to observe [[NGC 4993]], a galaxy {{convert|40|Mpc|Mly|abbr=on|lk=on}} away in the [[Hydra (constellation)|Hydra constellation]].<ref name="NAT-20170825">{{cite news |last=Castelvecchi |first=Davide |title=Rumours swell over new kind of gravitational-wave sighting |date=August 2017 |journal=[[Nature (journal)|Nature News]] |doi=10.1038/nature.2017.22482}}</ref><ref name="NS-20170823">{{cite news |last=McKinnon |first=Mika |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 |work=[[New Scientist]] |accessdate=28 August 2017 }}</ref> The collaboration had earlier declined to comment on the rumors, not adding to a previous announcement that there were several triggers under analysis.<ref name="LIGO-20170825">{{cite news
The first public information about the event was tweeted by astronomer J. Craig Wheeler of the [[University of Texas at Austin]] on 18 August 2017. He later deleted the tweet and apologized for [[Scoop (news)|scooping]] the official announcement protocol. Other people followed up on the rumor, and reported that the public logs of several major telescopes listed priority interruptions in order to observe [[NGC 4993]], a galaxy {{convert|40|Mpc|Mly|abbr=on|lk=on}} away in the [[Hydra (constellation)|Hydra constellation]].<ref name="NAT-20170825">{{cite news |last=Castelvecchi |first=Davide |title=Rumours swell over new kind of gravitational-wave sighting |date=August 2017 |journal=[[Nature (journal)|Nature News]] |doi=10.1038/nature.2017.22482}}</ref><ref name="NS-20170823">{{cite news |last=McKinnon |first=Mika |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 |work=[[New Scientist]] |accessdate=28 August 2017 }}</ref> The collaboration had earlier declined to comment on the rumors, not adding to a previous announcement that there were several triggers under analysis.<ref name="LIGO-20170825">{{cite news
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==Gamma ray detection==
==Gamma ray detection==
[[File:Artist NSIllustration CREDIT NSF LIGO Sonoma State University A. Simonnet.jpg|thumb|150px|Artistic concept: two neutron stars merge.]]
[[File:Artist NSIllustration CREDIT NSF LIGO Sonoma State University A. Simonnet.jpg|thumb|150px|Artistic concept: two neutron stars merge.]]
The first electromagnetic signal detected was [[GRB 170817A]], a [[Gamma-ray burst#Short gamma-ray bursts|short gamma ray burst]], detected {{val|1.74|0.05|u=seconds}} after the merger time and lasting less than two seconds.<ref name="NASA-20171016" /><ref name="NAT-20170825" />
The first electromagnetic signal detected was [[GRB 170817A]], a [[Gamma-ray burst#Short gamma-ray bursts|short gamma ray burst]], detected {{val|1.74|0.05|u=seconds}} after the merger time and lasting for a few seconds.{{r|MN-20171016|NAT-20170825}}


GRB 170817A was discovered by the [[Fermi Gamma-ray Space Telescope|Fermi]] Gamma Ray Telescope, with an automatic alert issued just 14 seconds after the GRB detection. After the LIGO/Virgo circular 40 minutes later, manual processing of data from the [[INTEGRAL]] gamma-ray telescope also detected the same GRB. The difference in arrival time between Fermi and INTEGRAL helped to improve the sky localization of the gamma-ray emission.
GRB 170817A was discovered by the [[Fermi Gamma-ray Space Telescope|Fermi]] gamma-ray telescope, with an automatic alert issued just 14 seconds after the GRB detection. After the LIGO/Virgo circular 40 minutes later, manual processing of data from the [[INTEGRAL]] gamma-ray telescope also detected the same GRB. 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 [[NGC 4993]], possibly due to its [[Astrophysical jet|jet]]s not being pointed directly toward Earth, but rather at an angle of about 30 degrees to the side.<ref name="NASA-20171016" /><ref name=":0" /><ref>{{Cite news|url=https://www.washingtonpost.com/news/speaking-of-science/wp/2017/10/17/what-the-new-gravitational-waves-discovery-means-for-the-future-of-astronomy/|title=Analysis {{!}} What the new gravitational waves discovery means for the future of astronomy|last=Kaplan|first=Sarah|date=2017-10-17|work=Washington Post|access-date=2017-10-29|language=en-US|issn=0190-8286}}</ref>
This GRB was relatively faint given the proximity of the host galaxy [[NGC 4993]], possibly due to its [[Astrophysical jet|jet]]s not being pointed directly toward Earth, but rather at an angle of about 30 degrees to the side.{{r|SM-20171016}}<ref name=spacecom>{{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
|first1=Charles Q. |last1=Choi
|date=16 October 2017 |website=[[Space.com]] |publisher=[[Purch Group]]
|access-date=16 October 2017
}}</ref>


==Electromagnetic follow-up==
==Electromagnetic follow-up==
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}}</ref> These prompted a massive search by many [[Astronomical survey|survey]] and [[robotic telescope]]s. In addition to its expected large size (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 only an hour after [[twilight]] for any given telescope.
}}</ref> These prompted a massive search by many [[Astronomical survey|survey]] and [[robotic telescope]]s. In addition to its expected large size (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 only an hour after [[twilight]] for any given telescope.


In total six teams (SSS, DLT40, [[VISTA (telescope)|VISTA]], MASTER, [[DECam]], [[Las Cumbres Observatory]] (LCO) Chile) imaged the same new source independently in a 90-minute interval.<ref name="APJ" /> The first to detect optical light associated with the collision was the Swope Supernova Survey, which found it in an image of [[NGC 4993]] taken 10 hours and 52 minutes after the event<ref name="APJ" /> by the {{convert|1|meter}} diameter Swope Telescope operating in the optical at [[Las Campanas Observatory]], Chile. They were also the first to announce it, naming their detection SSS17a. The new source was later given an official [[International Astronomical Union]] (IAU) designation of [[AT 2017gfo]].
In total six teams (SSS, DLT40, [[VISTA (telescope)|VISTA]], Master, [[DECam]], [[Las Cumbres Observatory]] (LCO) 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 Swope Supernova Survey, which found it in an image of [[NGC 4993]] taken 10 hours and 52 minutes after the event{{r|MN-20171016|APJ|Drout}} by the {{convert|1|meter}} diameter 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 12h 26min post-event. The new source was later given an official [[International Astronomical Union]] (IAU) designation of [[AT 2017gfo]].

The SSS 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 a more accurate distance than based on gravitational waves alone.


The detection of the optical/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.
The detection of the optical/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. Over the following days, the color of the optical source changed from blue to red as the source expanded and cooled.
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.
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.


Nine days later, the source was detected in X-rays by the [[Chandra X-ray Observatory]]<ref name="NASA-20171016" />. Sixteen days after the merger event, the source was detected in radio with the [[Karl G. Jansky Very Large Array]] (VLA) in [[New Mexico]].{{r|SM-20171016}} More than 70 observatories covering the [[electromagnetic spectrum]] observed the event.{{r|SM-20171016}}
Nine days later, the source was detected in X-rays by the [[Chandra X-ray Observatory]] (after non-detections at earlier times). Sixteen days after the merger event, the source was detected in radio with the [[Karl G. Jansky Very Large Array]] (VLA) in [[New Mexico]].{{r|SM-20171016}} More than 70 observatories covering the [[electromagnetic spectrum]] observed the event.{{r|SM-20171016}}


There are multiple strong lines of evidence that AT 2017gfo is indeed the aftermath of GW 170817: the colour 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 pre-event show nothing at the location of AT 2017gfo, ruling out a foreground variable star in the Milky Way.{{r|APJ}}
There are multiple strong lines of evidence that AT 2017gfo is indeed the aftermath of GW 170817: the colour 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 pre-event show nothing at the location of AT 2017gfo, ruling out a foreground variable star in the Milky Way.{{r|APJ}}
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The [[chirp mass]], a directly observable parameter which may be very roughly equated to the geometric mean of the masses, is measured at {{val|1.188|0.004|0.002|u=solar mass}}.{{r|Abbott^3_AJT}}
The [[chirp mass]], a directly observable parameter which may be very roughly equated to the geometric mean of the masses, is measured at {{val|1.188|0.004|0.002|u=solar mass}}.{{r|Abbott^3_AJT}}


The neutron star merger event is thought to result in a [[kilonova]], characterized by a short [[gamma ray burst]] followed by a longer optical "afterglow" powered by the [[radioactive decay]] of heavy [[r-process]] nuclei. Kilonovae are candidates for the production of half the chemical elements heavier than iron in the Universe.{{r|SM-20171016}} A total of 16,000 times the [[Earth mass|mass of the Earth]] in heavy elements is believed to have formed, including approximately ten [[Earth mass]]es just of the two elements gold and platinum.
The neutron star merger event is thought to result in a [[kilonova]], characterized by a short [[gamma ray burst]] followed by a longer optical "afterglow" powered by the [[radioactive decay]] of heavy [[r-process]] nuclei. Kilonovae are candidates for the production of half the chemical elements heavier than iron in the Universe.{{r|SM-20171016}} A total of 16,000 times the [[Earth mass|mass of the Earth]] in heavy elements is believed to have formed, including approximately ten [[Earth mass]]es just of the two elements gold and platinum.<ref name=EdoBerger>{{cite AV media|first = Edo|last = Berger|time = 1 h 48 min|url = https://www.youtube.com/watch?v=mtLPKYl4AHs&feature=youtu.be|title = LIGO/Virgo Press Conference|date = 16 October 2017|accessdate = 29 October 2017}}</ref>


It is not known what object was produced by the merger. It could be either a neutron star heavier than any known neutron star, or a [[black hole]] lighter than any known black hole.
It is not known what object was produced by the merger. It could be either a neutron star heavier than any known neutron star, or a [[black hole]] lighter than any known black hole.{{r|spacecom}}


==Scientific importance==
==Scientific importance==
Scientific interest in the event was enormous, with dozens of preliminary papers (and almost 100 [[Manuscript (publishing)#Preprint|preprints]]<ref>{{cite web |title=ArXiv.org search for GW170817 |url=https://arxiv.org/find/all/1/all:+GW170817/0/1/0/all/0/1 |accessdate=18 October 2017}}</ref>) published the day of the announcement, including eight letters in ''[[Science (journal)|Science]]'',{{r|SM-20171016}} six in ''[[Nature (journal)|Nature]]''<ref>{{Cite journal|last=Miller|first=M. Coleman|date=2017-10-16|title=Gravitational waves: A golden binary|url=https://www.nature.com/nature/journal/vaop/ncurrent/full/nature24153.html|journal=Nature|language=en|volume=advance online publication|doi=10.1038/nature24153|issn=1476-4687}}</ref>, and 23 in a special issue of ''[[The Astrophysical Journal Letters]]'' devoted to the subject.
Scientific interest in the event was enormous, with dozens of preliminary papers (and almost 100 [[Manuscript (publishing)#Preprint|preprints]]<ref>{{cite web |title=ArXiv.org search for GW170817 |url=https://arxiv.org/find/all/1/all:+GW170817/0/1/0/all/0/1 |accessdate=18 October 2017}}</ref>) published the day of the announcement, including eight letters in ''[[Science (journal)|Science]]'',{{r|SM-20171016}} six in ''[[Nature (journal)|Nature]]'', and 23 in a special issue of ''[[The Astrophysical Journal Letters]]'' devoted to the subject.<ref name=ApJL>{{cite journal |title=Focus on the Electromagnetic Counterpart of the Neutron Star Binary Merger GW170817 |journal=[[The Astrophysical Journal Letters]] |volume=848 |issue=2 |first=Edo |last=Berger |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).}}</ref>


This is not the first observation that is known to be of a neutron star merger; [[GRB 130603B]] was the first observed [[kilonova]]. It is however, by far the ''best'' observation, making this the strongest evidence to date to confirm the hypothesis that mergers of binary stars are the cause of short gamma-ray bursts.{{r|APJ}}{{r|PhysRev2017}}
This is not the first observation that is known to be of a neutron star merger; [[GRB 130603B]] was the first observed [[kilonova]]. It is however, by far the ''best'' observation, making this the strongest evidence to date to confirm the hypothesis that mergers of binary stars are the cause of short gamma-ray bursts.{{r|APJ}}{{r|PhysRev2017}}

Revision as of 21:03, 29 October 2017

GW170817
The GW170817 signal as measured by the LIGO and Virgo gravitational wave detectors. Signal is invisible in the Virgo data
Event typeGravitational wave event Edit this on Wikidata
InstrumentLIGO, Virgo
Right ascension13h 09m 48.08s[1]
Declination−23° 22′ 53.3″[1]
EpochJ2000.0
Distance40 megaparsecs (130 Mly)
Redshift0.0099 Edit this on Wikidata
Other designationsGW170817
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[edit on Wikidata]

GW170817 is a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 17 August 2017. The GW was produced by the last minutes of two neutron stars spiralling closer to each other and finally merging, and is the first time a GW observation has been confirmed by non-gravitational means.

Unlike all previous GW detections, which were of merging black holes not expected to produce a detectable electromagnetic signal,[2][3][a] the aftermath of this merger was also seen by many conventional telescopes, as well as gamma-ray detection satellites, marking a significant breakthrough for multi-messenger astronomy.[5][6][7][8][9]

Technically, there were three separate observations, and strong evidence that they came from the same astronomical source:

  • The GW signal, which had a duration of approximately 100 seconds, shows the characteristics in intensity and frequency expected of the inspiral of two neutron stars. Analysis of the slight variation in arrival time of the GW at the three detector locations (two LIGO and one Virgo) yielded an approximate angular direction to the source.
  • The short gamma-ray burst GRB 170817A[10][11] detected by the Fermi and INTEGRAL spacecraft 1.7 seconds after the GW signal ended. These detectors have very limited directional sensitivity, but indicated a large area of the sky which overlapped the gravitational wave position. It has long been theorized that short gamma-ray bursts are caused by neutron star mergers.
  • The optical transient AT 2017gfo, found 11 hours later in the galaxy NGC 4993[12] during a search of the region indicated by the GW detection. This was observed by numerous telescopes, from radio to X-ray wavelengths, over the following days and weeks, and shows the characteristics (a fast-moving, rapidly-cooling cloud of neutron-rich material) expected of debris ejected from a neutron-star merger.

Announcement

It’s the first time that we’ve observed a cataclysmic astrophysical event in both gravitational waves and electromagnetic waves — our cosmic messengers.[13]

David Reitze, LIGO executive director

The event was officially announced on 16 October 2017[10][11] at press conferences at the National Press Club in Washington, D.C. and at the ESO headquarters in Garching bei München in Germany.[12]

The first public information about the event was tweeted by astronomer J. Craig Wheeler of the University of Texas at Austin on 18 August 2017. He later deleted the tweet and apologized for scooping the official announcement protocol. Other people followed up on the rumor, and reported that the public logs of several major telescopes listed priority interruptions in order to observe NGC 4993, a galaxy 40 Mpc (130 Mly) away in the Hydra constellation.[14][15] The collaboration had earlier declined to comment on the rumors, not adding to a previous announcement that there were several triggers under analysis.[16][17]

Gravitational wave detection

Artist's impression 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 starting from a frequency of 24 hertz (cycles per second). It covered approximately 3000 cycles, increasing in amplitude and frequency to a few hundred Hz in the typical inspiral chirp pattern, ending with the collision received at 12:41:04.4 UTC. It arrived first at the Virgo detector in Italy, then 22 milliseconds later at the LIGO-Livingston detector in Louisiana, USA, and another 3 milliseconds later at the LIGO-Hanford detector in the state of Washington, USA.[18] The signal was detected and analyzed by a comparison with a template (i.e. the prediction from general relativity) defined from the post-Newtonian approximation.[19]

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 (14 sec post-event), 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 followup teams at 40 minutes post-event.[20]

Sky localisation of the event requires combining data from the three interferometers; this was delayed by two problems. The Virgo data were delayed by a data transmission problem, and the LIGO Livingston data were corrupted by a glitch (brief burst of instrument noise) a few seconds before the climax. These required manual analysis before the sky location could be announced about 4.5 hours post-event.[21] The three detections localized the source to an area of 28 square degrees in the southern sky with a 90% probability.[5]

Gamma ray detection

Artistic concept: two neutron stars merge.

The first electromagnetic signal detected was GRB 170817A, a short gamma ray burst, detected 1.74±0.05 s after the merger time and lasting for a few seconds.[11][14]

GRB 170817A was discovered by the Fermi gamma-ray telescope, with an automatic alert issued just 14 seconds after the GRB detection. After the LIGO/Virgo circular 40 minutes later, manual processing of data from the INTEGRAL gamma-ray telescope also detected the same GRB. 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 NGC 4993, possibly due to its jets not being pointed directly toward Earth, but rather at an angle of about 30 degrees to the side.[12][22]

Electromagnetic follow-up

Hubble picture of NGC 4993 with inset showing GRB170817A over 6 days. Credit: NASA and ESA
Optical lightcurves
The change in optical and near-infrared spectra

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, and a three-detector sky location at 17:54 UTC.[20] These prompted a massive search by many survey and robotic telescopes. In addition to its expected large size (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 only an hour after twilight for any given telescope.

In total six teams (SSS, DLT40, VISTA, Master, DECam, Las Cumbres Observatory (LCO) Chile) imaged the same new source independently in a 90-minute interval.[1]: 5  The first to detect optical light associated with the collision was the Swope Supernova Survey, which found it in an image of NGC 4993 taken 10 hours and 52 minutes after the event[11][1][23] by the 1 meter (3 ft 3 in) diameter 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 12h 26min post-event. The new source was later given an official International Astronomical Union (IAU) designation of AT 2017gfo.

The SSS team surveyed all galaxies in the region of space predicted by the gravitational wave observations, and identified a single new transient.[22][23] By identifying the host galaxy of the merger, it is possible to provide a more accurate distance than based on gravitational waves alone.

The detection of the optical/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.[23] Over the following days, the color of the optical source changed from blue to red as the source expanded and cooled.[22]

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.

Nine days later, the source was detected in X-rays by the Chandra X-ray Observatory (after non-detections at earlier times). Sixteen days after the merger event, the source was detected in radio with the Karl G. Jansky Very Large Array (VLA) in New Mexico.[12] More than 70 observatories covering the electromagnetic spectrum observed the event.[12]

There are multiple strong lines of evidence that AT 2017gfo is indeed the aftermath of GW 170817: the colour 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 pre-event show nothing at the location of AT 2017gfo, ruling out a foreground variable star in the Milky Way.[1]

Other detectors

No neutrinos consistent with the source were found in follow-up searches.[5][1] 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.[24][25]

Astrophysical origin and products

The gravitational wave signal indicated that the gravitational wave event was associated with the collision of two neutron stars[14][15][17][26] with a total mass of 2.82+0.47
−0.09 times the mass of the sun (solar masses).[5] If low spins are assumed, consistent with those observed in binary neutron stars that will merge within a Hubble time, the total mass is 2.74+0.04
−0.01 M☉.

The masses of the component stars have greater uncertainty. The larger (m1) has a 90% chance of being between 1.36 and 2.26 M☉, and the smaller (m2) has a 90% chance of being between 0.86 and 1.36 M☉.[27] Under the low spin assumption, the ranges are 1.36 to 1.60 M☉ for m1 and 1.17 to 1.36 M☉ for m2.

The chirp mass, a directly observable parameter which may be very roughly equated to the geometric mean of the masses, is measured at 1.188+0.004
−0.002 M☉.[27]

The neutron star merger event is thought to result in a kilonova, characterized by a short gamma ray burst followed by a longer optical "afterglow" powered by the radioactive decay of heavy r-process nuclei. Kilonovae are candidates for the production of half the chemical elements heavier than iron in the Universe.[12] A total of 16,000 times the mass of the Earth in heavy elements is believed to have formed, including approximately ten Earth masses just of the two elements gold and platinum.[28]

It is not known what object was produced by the merger. It could be either a neutron star heavier than any known neutron star, or a black hole lighter than any known black hole.[22]

Scientific importance

Scientific interest in the event was enormous, with dozens of preliminary papers (and almost 100 preprints[29]) published the day of the announcement, including eight letters in Science,[12] six in Nature, and 23 in a special issue of The Astrophysical Journal Letters devoted to the subject.[6]

This is not the first observation that is known to be of a neutron star merger; GRB 130603B was the first observed kilonova. It is however, by far the best observation, making this the strongest evidence to date to confirm the hypothesis that mergers of binary stars are the cause of short gamma-ray bursts.[1][5]

The event also provides 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, vGW − vEM, is constrained to between −3×10−15 and +7×10−16 times the speed of light.[27] In addition, it allowed investigation of the equivalence principle (through Shapiro delay measurement) and Lorentz invariance.[5] 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.[27] GW170817 also excluded some alternatives to general relativity, including variants of scalar–tensor theory[30][31][32][33], Hořava–Lifshitz gravity[34][35][36] and bimetric gravity.[37]

Gravitational wave signals such as GW170817 may be used as a standard siren to provide an independent measurement of the Hubble constant.[38][39]

Electromagnetic observations helped to support the theory that the mergers of neutron stars contribute to rapid neutron capture r-process nucleosynthesis[23] and are significant sources of r-process elements heavier than iron,[1] including gold and platinum.[28]

See also

Notes

  1. ^ Although acknowledged as unlikely, several mechanisms have been suggested by which a black hole merger could be surrounded by sufficient matter to produce an electromagnetic signal, which astronomers have been searching for.[3][4]

References

  1. ^ a b c d e f g h Abbott, B. P.; et al. (LIGO, Virgo and others collaboration) (October 2017). "Multi-messenger Observations of a Binary Neutron Star Merger" (PDF). The Astrophysical Journal. 848 (2): L12. arXiv:1710.05833. doi:10.3847/2041-8213/aa91c9. {{cite journal}}: Unknown parameter |class= ignored (help)
  2. ^ Connaughton, Valerie (2016). "Focus on Electromagnetic Counterparts to Binary Black Hole Mergers". The Astrophysical Journal Letters (Introduction). 826 (1). The follow-up observers sprang into action, not expecting to detect a signal if the gravitational radiation was indeed from a binary black-hole merger. [...] most observers and theorists agreed: the presence of at least one neutron star in the binary system was a prerequisite for the production of a circumbinary disk or neutron star ejecta, without which no electromagnetic counterpart was expected.
  3. ^ a b Loeb, Abraham (March 2016). "Electromagnetic Counterparts to Black Hole Mergers Detected by LIGO". The Astrophysical Journal Letters. 819 (2): L21. arXiv:1602.04735. Bibcode:2016ApJ...819L..21L. doi:10.3847/2041-8205/819/2/L21. Mergers of stellar-mass black holes (BHs) [...] are not expected to have electromagnetic counterparts. [...] I show that the [GW and gamma-ray] signals might be related if the BH binary detected by LIGO originated from two clumps in a dumbbell configuration that formed when the core of a rapidly rotating massive star collapsed. {{cite journal}}: Unknown parameter |class= ignored (help)
  4. ^ de Mink, S.E.; King, A. (April 2017). "Electromagnetic Signals Following Stellar-mass Black Hole Mergers" (PDF). The Astrophysical Journal Letters. 839 (1): L7. arXiv:1703.07794. Bibcode:2017ApJ...839L...7D. doi:10.3847/2041-8213/aa67f3. It is often assumed that gravitational-wave (GW) events resulting from the merger of stellar-mass black holes are unlikely to produce electromagnetic (EM) counterparts. We point out that the progenitor binary has probably shed a mass ≳10 M☉ during its prior evolution. If even a tiny fraction of this gas is retained in a circumbinary disk, the sudden mass loss and recoil of the merged black hole shocks and heats it within hours of the GW event. Whether the resulting EM signal is detectable is uncertain. {{cite journal}}: Unknown parameter |class= ignored (help)CS1 maint: unflagged free DOI (link)
  5. ^ a b c d e f Abbott, B. P.; et al. (LIGO Scientific Collaboration & Virgo Collaboration) (October 2017). "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral" (PDF). Physical Review Letters. 119 (16). arXiv:1710.05832. doi:10.1103/PhysRevLett.119.161101. {{cite journal}}: Unknown parameter |class= ignored (help)
  6. ^ a b Berger, Edo (16 October 2017). "Focus on the Electromagnetic Counterpart of the Neutron Star Binary Merger GW170817". The Astrophysical Journal Letters (Editorial). 848 (2). 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).
  7. ^ Landau, Elizabeth; Chou, Felicia; Washington, Dewayne; Porter, Molly (16 October 2017). "NASA Missions Catch First Light from a Gravitational-Wave Event". NASA. Retrieved 16 October 2017.
  8. ^ Botkin-Kowacki, Eva (16 October 2017). "Neutron star discovery marks breakthrough for 'multi-messenger astronomy'". The Christian Science Monitor. Retrieved 17 October 2017.
  9. ^ Metzger, Brian D. (16 October 2017). "Welcome to the Multi-Messenger Era! Lessons from a Neutron Star Merger and the Landscape Ahead". arXiv:1710.05931 [astro-ph.HE].
  10. ^ a b Overbye, Dennis (16 October 2017). "LIGO Detects Fierce Collision of Neutron Stars for the First Time". The New York Times. Retrieved 16 October 2017.
  11. ^ a b c d Krieger, Lisa M. (16 October 2017). "A Bright Light Seen Across The Universe, Proving Einstein Right - Violent collisions source of our gold, silver". The Mercury News. Retrieved 16 October 2017.
  12. ^ a b c d e f g Cho, Adrian (October 2017). "Merging neutron stars generate gravitational waves and a celestial light show". Science. doi:10.1126/science.aar2149.
  13. ^ "LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars". MIT News. 16 October 2017. Retrieved 23 October 2017.
  14. ^ a b c Castelvecchi, Davide (August 2017). "Rumours swell over new kind of gravitational-wave sighting". Nature News. doi:10.1038/nature.2017.22482.
  15. ^ a b McKinnon, Mika (23 August 2017). "Exclusive: We may have detected a new kind of gravitational wave". New Scientist. Retrieved 28 August 2017.
  16. ^ Staff (25 August 2017). "A very exciting LIGO-Virgo Observing run is drawing to a close August 25". LIGO. Retrieved 29 August 2017.
  17. ^ a b Drake, Nadia (25 August 2017). "Strange Stars Caught Wrinkling Spacetime? Get the Facts". National Geographic. Retrieved 27 August 2017.
  18. ^ Kohler, Susanna (16 October 2017). "Neutron-Star Merger Detected By Many Eyes and Ears". AAS Nova. Retrieved 16 October 2017. {{cite web}}: Italic or bold markup not allowed in: |publisher= (help)
  19. ^ Blanchet, Luc (2014). "Gravitational radiation from post-Newtonian sources and inspiralling compact binaries". Living Reviews in Relativity. 17 (2). doi:10.12942/lrr-2014-2.
  20. ^ a b "GCN circulars related to LIGO trigger G298048". Gamma-ray Burst Coordinates Network. Goddard Space Flight Center, NASA. 17 August 2017. Retrieved 19 October 2017.
  21. ^ Berry, Christopher (16 October 2017). "GW170817—The pot of gold at the end of the rainbow". Retrieved 19 October 2017.
  22. ^ a b c d Choi, Charles Q. (16 October 2017). "Gravitational Waves Detected from Neutron-Star Crashes: The Discovery Explained". Space.com. Purch Group. Retrieved 16 October 2017.
  23. ^ a b c d Drout, M. R.; et al. (October 2017). "Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesis" (PDF). Science. arXiv:1710.05443. doi:10.1126/science.aaq0049. {{cite journal}}: Unknown parameter |class= ignored (help)
  24. ^ Albert, A.; et al. (Antares Collaboration, IceCube Collaboration, Pierre Auger Collaboration, LIGO Scientific Collaboration, & Virgo Collaboration) (October 2017). "Search for High-energy Neutrinos from Binary Neutron Star Merger GW170817 with ANTARES, IceCube, and the Pierre Auger Observatory" (PDF). arXiv:1710.05839. {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |class= ignored (help)
  25. ^ Bravo, Sylvia (16 October 2016). "No neutrino emission from a binary neutron star merger". IceCube South Pole Neutrino Observatory. Retrieved 20 October 2017.
  26. ^ Sokol, Joshua (25 August 2017). "What Happens When Two Neutron Stars Collide? Scientific Revolution". Wired. Retrieved 27 August 2017.
  27. ^ a b c d Abbott, B. P.; et al. (2017). "Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A". The Astrophysical Journal Letters. 848 (2): L13. arXiv:1710.05834. doi:10.3847/2041-8213/aa920c. {{cite journal}}: Unknown parameter |class= ignored (help)
  28. ^ a b Berger, Edo (16 October 2017). LIGO/Virgo Press Conference. Event occurs at 1 h 48 min. Retrieved 29 October 2017.
  29. ^ "ArXiv.org search for GW170817". Retrieved 18 October 2017.
  30. ^ Lombriser, Lucas; Taylor, Andy (28 September 2015). "Breaking a Dark Degeneracy with Gravitational Waves". arXiv:1509.08458 [astro-ph.CO].
  31. ^ Lombriser, Lucas; Lima, Nelson (24 February 2016). "Challenges to Self-Acceleration in Modified Gravity from Gravitational Waves and Large-Scale Structure". arXiv:1602.07670 [astro-ph.CO].
  32. ^ "Quest to settle riddle over Einstein's theory may soon be over". phys.org. 10 February 2017. Retrieved 29 October 2017.
  33. ^ "Theoretical battle: Dark energy vs. modified gravity". Ars Technica. 25 February 2017. Retrieved 27 October 2017.
  34. ^ Creminelli, Paolo; Vernizzi, Filippo (16 October 2017). "Dark Energy after GW170817". arXiv:1710.05877 [astro-ph.CO].
  35. ^ Sakstein, Jeremy; Jain, Bhuvnesh (16 October 2017). "Implications of the Neutron Star Merger GW170817 for Cosmological Scalar-Tensor Theories". arXiv:1710.05893 [astro-ph.CO].
  36. ^ Ezquiaga, Jose María; Zumalacárregui, Miguel (16 October 2017). "Dark Energy after GW170817". arXiv:1710.05901 [astro-ph.CO].
  37. ^ Baker, T.; Bellini, E.; Ferreira, P.G.; Lagos, M.; Noller, J.; Sawicki, I. (19 October 2017). "Strong constraints on cosmological gravity from GW170817 and GRB 170817A". arXiv:1710.06394 [astro-ph.CO].
  38. ^ Abbott, B. P.; et al. (LIGO Scientific Collaboration & Virgo Collaboration) (16 October 2017). "A gravitational-wave standard siren measurement of the Hubble constant". Nature. arXiv:1710.05835. doi:10.1038/nature24471. {{cite journal}}: Unknown parameter |class= ignored (help)
  39. ^ Scharping, Nathaniel (18 October 2017). "Gravitational Waves Show How Fast The Universe is Expanding". Astronomy. Retrieved 18 October 2017.

External links

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