View source for GW190521

{{short description|Gravitational wave observation}}
{{Use dmy dates|date=June 2020}}
{{Infobox astronomical event
 |image_size=280px
|caption=The GW event GW190521 observed by the LIGO Hanford (left), LIGO Livingston (middle), and Virgo (right) detectors
 |ra={{RA|12|49|42.3}}<ref name="PRL-20200625"/>
 |dec={{DEC|−34|49|29}}<ref name="PRL-20200625"/>
 |epoch=[[J2000.0]]
 |distance={{convert|5300|Mpc|lk=on}}<ref name="AJ-20200902" />
 |detected_by=[[LIGO]], [[Virgo interferometer|Virgo]]<ref name="LIGO-20190521" /><ref name="NASA-20200625" />
}}
'''GW190521''' (initially '''S190521g''')<ref name="PRL-20200902" /> was a [[gravitational wave]] signal resulting from the merger of two [[black hole]]s. It was possibly associated with a coincident flash of light; if this association is correct, the merger would have occurred near a third [[supermassive black hole]].<ref name="NASA-20200625" /><ref name="NYT-20200625" /> The event was observed by the [[LIGO]] and [[Virgo interferometer|Virgo]] detectors on 21 May 2019 at 03:02:29 UTC,<ref name="UL-2020">{{cite news |title=GW trigger S190521g ('GW 190521') |url=https://www.swift.ac.uk/LVC/S190521g/ |date=2020 |work=[[University of Leicester]] |access-date=26 June 2020 |archive-date=28 June 2020 |archive-url=https://web.archive.org/web/20200628111356/https://www.swift.ac.uk/LVC/S190521g/ |url-status=live }}</ref> and published on 2 September 2020.<ref name="AJ-20200902" /><ref name="PRL-20200902" /><ref name="LIGO-20200902" /> The event had a [[Distance_measure#Luminosity_distance|Luminosity distance]] of 17&nbsp;billion light years away from Earth,{{refn|group=note|name=firs|"The event unfolded at an almost unimaginable distance from Earth — in a spot that is now 17 billion light-years away according to standard cosmological calculations that describe an expanding universe."<ref name="NYT-20200903" />}}<ref name="PRL-20200902"/><ref name="NYT-20200903" /> within a 765 deg<sup>2</sup> area<ref group=note>The relatively large and distant area of the sky within which it is claimed to be possible to localize the source.</ref><ref name="NASA-20190521">{{cite web |last=Mo |first=Geoffrey |title=GCN Circular - Number: 24640 - LIGO/Virgo S190521g: Updated sky localization |url=https://gcn.gsfc.nasa.gov/other/GW190521g.gcn3 |date=21 May 2020 |work=[[NASA]] |access-date=27 June 2020 |archive-date=27 June 2020 |archive-url=https://web.archive.org/web/20200627180941/https://gcn.gsfc.nasa.gov/other/GW190521g.gcn3 |url-status=live }}</ref> towards [[Coma Berenices]], [[Canes Venatici]], or [[Phoenix (constellation)|Phoenix]].<ref name="LIGO-20190521">{{cite web |title=Superevent info - S190521g |url=https://gracedb.ligo.org/superevents/S190521g/view/ |date=21 May 2019 |work=[[LIGO]] |access-date=25 June 2020 |archive-date=28 June 2020 |archive-url=https://web.archive.org/web/20200628043832/https://gracedb.ligo.org/superevents/S190521g/view/ |url-status=live }}</ref><ref name="NASA-20200625">{{cite news |last=Cofield |first=Calla |title=Black Hole Collision May Have Exploded With Light |url=https://www.jpl.nasa.gov/news/news.php?feature=7688 |date=25 June 2020 |work=[[NASA]] |access-date=25 June 2020 |archive-date=28 June 2020 |archive-url=https://web.archive.org/web/20200628032649/https://www.jpl.nasa.gov/news/news.php?feature=7688 |url-status=live }}</ref><ref name="NYT-20200625">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Two Black Holes Colliding Not Enough? Make It Three - Astronomers claim to have seen a flash from the merger of two black holes within the maelstrom of a third, far bigger one. |url=https://www.nytimes.com/2020/06/25/science/black-hole-collision-ligo.html |date=25 June 2020 |work=[[The New York Times]] |access-date=25 June 2020 |archive-date=25 June 2020 |archive-url=https://web.archive.org/web/20200625153002/https://www.nytimes.com/2020/06/25/science/black-hole-collision-ligo.html |url-status=live }}</ref><ref name="PHYS-20200625">{{cite news |author=Graduate Center, CUNY |title=Black hole collision may have exploded with light |url=https://phys.org/news/2020-06-black-hole-collision.html |date=25 June 2020 |work=[[Phys.org]] |access-date=26 June 2020 |archive-date=26 June 2020 |archive-url=https://web.archive.org/web/20200626004744/https://phys.org/news/2020-06-black-hole-collision.html |url-status=live }}</ref>

At 85 and 66 [[solar masses]] (<var>M</var><sub>☉</sub>) respectively, the two black holes comprising this merger are the largest progenitor masses observed to date.<ref name="FRBS-20200903">{{cite news |last=Siegel |first=Ethan |title=LIGO's Biggest Mass Merger Ever Foretells A Black Hole Revolution |url=https://www.forbes.com/sites/startswithabang/2020/09/03/ligos-biggest-mass-merger-ever-foretells-a-black-hole-revolution/ |date=3 September 2020 |work=[[Forbes]] |access-date=5 September 2020 |archive-date=4 September 2020 |archive-url=https://web.archive.org/web/20200904074001/https://www.forbes.com/sites/startswithabang/2020/09/03/ligos-biggest-mass-merger-ever-foretells-a-black-hole-revolution/ |url-status=live }}</ref> The resulting black hole had a mass equivalent to 142 times that of the Sun, making this the first clear detection of an [[intermediate-mass black hole]]. The remaining 9 solar masses were radiated away as energy in the form of gravitational waves.<ref name="AJ-20200902">{{cite journal |author=Abbott, R. |display-authors=et al. |title=Properties and Astrophysical Implications of the 150 M ⊙ Binary Black Hole Merger GW190521 |journal=[[The Astrophysical Journal]] |date=2 September 2020 |volume=900 |issue=1 |pages=L13 |doi=10.3847/2041-8213/aba493 |arxiv=2009.01190 |bibcode=2020ApJ...900L..13A |doi-access=free}}</ref><ref name="PRL-20200902">{{cite journal |author=Abbott, R. |display-authors=et al. |title=GW190521: A Binary Black Hole Merger with a Total Mass of 150 M ⊙ |journal=[[Physical Review Letters]] |date=2 September 2020 |volume=125 |issue=10 |pages=101102 |doi=10.1103/PhysRevLett.125.101102 |pmid=32955328 |arxiv=2009.01075 |bibcode=2020PhRvL.125j1102A |doi-access=free}}</ref><ref name="LIGO-20200902">{{cite news |author=Martin |title=GW190521: The Most Massive Black Hole collision Observed To Date |url=https://www.ligo.org/science/Publication-GW190521/flyer.pdf |date=2 September 2020 |work=[[LIGO Scientific Collaboration]] |access-date=2 September 2020 |archive-date=4 September 2020 |archive-url=https://web.archive.org/web/20200904170948/https://www.ligo.org/science/Publication-GW190521/flyer.pdf |url-status=live }}</ref>

==Physical significance==
GW190521 is a significant discovery due to the masses of the resulting large black hole and of one or both of the smaller constituent black holes. [[Stellar evolution]] theory predicts that a star cannot collapse itself into a black hole of more than about {{solar mass|65}}, leaving a [[Stellar black hole#Upper mass gap|black hole mass gap]] above {{solar mass|65}}. The {{solar mass|{{val|85|21|14}}}}<ref group="note">This notation is used to state [[Uncertainty#In measurements|asymmetric uncertainty]].</ref> and {{solar mass|{{val|142|28|16}}}} black holes observed in GW190521 are conclusively in the mass gap, indicating that it can be populated by the mergers of smaller black holes.<ref name="AJ-20200902"/>

Only indirect evidence for intermediate mass black holes, those with between 100 and 100,000 [[solar mass]]es, had been observed earlier, and it was unclear how they had formed.<ref>{{cite journal |bibcode=2004IJMPD..13....1M|title=Intermediate-Mass Black Holes|last1=Miller|first1=M. Coleman|last2=Colbert|first2=E. J. M.|journal=International Journal of Modern Physics D|year=2004|volume=13|issue=1|page=1|doi=10.1142/S0218271804004426|arxiv=astro-ph/0308402|s2cid=118959484}}</ref> Researchers hypothesize that they form from a hierarchical series of mergers, in which each black hole is the result of successive mergers involving smaller black holes.<ref name="LIGO-20200902" />

According to discovery team member [[Vicky Kalogera|Vassiliki Kalogera]] of [[Northwestern University]], "this is the first and only firm/secure mass measurement of an intermediate mass black hole at the time of its birth ... Now we know reliably at least one way [such objects can form], through the merger of other black holes."<ref name="NYT-20200903">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |archive-url=https://web.archive.org/web/20200902193002/https://www.nytimes.com/2020/09/02/science/black-hole-astronomy-physics.html |archive-date=2 September 2020 |title=These Black Holes Shouldn't Exist, but There They Are - On the far side of the universe, a collision of dark giants sheds light on an invisible process of cosmic growth. |url=https://www.nytimes.com/2020/09/02/science/black-hole-astronomy-physics.html |date=3 September 2020 |work=[[The New York Times]] |access-date=4 September 2020 |url-status=live}}</ref>

== Possible electromagnetic counterpart==
In June 2020, astronomers reported observations of a flash of light that might be associated with GW190521. The [[Zwicky Transient Facility]] (ZTF) reported a transient optical source within the region of the GW190521 trigger, though as the uncertainty in sky position was hundreds of square degrees, the association remains uncertain. If the two events are actually linked, the event is claimed to be the first finding of an [[electromagnetic radiation|electromagnetic source]] related to the merger of two black holes.<ref name="NASA-20200625" /><ref name="PRL-20200625">{{cite journal |author=Graham, M.J. |display-authors=et al. |url=https://authors.library.caltech.edu/104019/1/PhysRevLett.124.251102.pdf |title=Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational-Wave Event S190521g |journal=[[Physical Review Letters]] |year=2020 |volume=124 |issue=25 |page=251102 |doi=10.1103/PhysRevLett.124.251102 |pmid=32639755 |arxiv=2006.14122 |bibcode=2020PhRvL.124y1102G |s2cid=220055995 |access-date=5 September 2020 |archive-date=11 July 2020 |archive-url=https://web.archive.org/web/20200711111646/https://authors.library.caltech.edu/104019/1/PhysRevLett.124.251102.pdf |url-status=live }}</ref><ref name="NYT-20200625" /><ref name="SD-20200626">{{cite news |title=Black hole collision may have exploded with light |url=https://www.spacedaily.com/reports/Black_hole_collision_may_have_exploded_with_light_999.html |date=26 June 2020 |work=[[ScienceDaily]] |access-date=26 June 2020 |archive-date=26 June 2020 |archive-url=https://web.archive.org/web/20200626151648/https://www.spacedaily.com/reports/Black_hole_collision_may_have_exploded_with_light_999.html |url-status=live }}</ref> Mergers of black holes do not typically emit any light. The researchers suggest that it could be explained if the merging of the two smaller black holes sent the newly formed intermediate mass black hole on a trajectory that hurtled through the [[accretion disk]] of an unrelated but nearby supermassive black hole, disrupting the disk material and producing a flare of light. The newly formed black hole would have traveled at {{convert|200|km/s|mi/s|abbr=on}} through the disk, according to the astronomers.<ref name="IFLS-20200625" /> If this explanation is correct, the flare should repeat after about 1.6 years<ref name="PRL-20200625" /> when the intermediate mass black hole again encounters the accretion disk.<ref name="IFLS-20200625" /> As of 2023, the status of the connection between these two events is unconfirmed.<ref>{{Cite journal |last1=Cui |first1=Zhe |last2=Li |first2=Xiang-Dong |date=2023-06-16 |title=On the formation of GW190521-like binary black hole merger systems |journal=Monthly Notices of the Royal Astronomical Society |volume=523 |issue=4 |pages=5565–5582 |doi=10.1093/mnras/stad1800 |doi-access=free |issn=0035-8711|arxiv=2306.08441 }}</ref>

According to Matthew Graham, lead astronomer for the study, "This supermassive black hole was burbling along for years before this more abrupt flare. The flare occurred on the right timescale, and in the right location, to be coincident with the gravitational-wave event. In our study, we conclude that the flare is likely the result of a black hole merger, but we cannot completely rule out other possibilities."<ref name="IFLS-20200625">{{cite news |last=Carpineti |first=Alfredo |title=Astronomers May Have Observed The First Flare From A Black Hole Collision |url=https://www.iflscience.com/space/astronomers-may-have-observed-the-first-flare-from-a-black-hole-collision/ |date=25 June 2020 |work=[[IFLScience]] |access-date=26 June 2020 |archive-date=29 June 2020 |archive-url=https://web.archive.org/web/20200629155728/https://www.iflscience.com/space/astronomers-may-have-observed-the-first-flare-from-a-black-hole-collision/ |url-status=live }}</ref>

== Possible eccentricity==
While the original [[LIGO]]/[[Virgo interferometer|Virgo]] data analysis assumed a quasi-circular inspiral waveform model, subsequent publications claimed that this source could have been significantly eccentric. Romero-Shaw et al. showed that the data is better described by a non-precessing eccentric waveform with <math>e_{\rm 10 Hz}\geq 0.1</math> than a spin-precessing quasi-circular model.<ref name="RomeroShaw">{{cite journal |author=Romero-Shaw, I. |display-authors=et al. |title=GW190521: Orbital Eccentricity and Signatures of Dynamical Formation in a Binary Black Hole Merger Signal |journal=[[The Astrophysical Journal]] |date=23 October 2020 |volume=903 |issue=1 |pages=5 |doi=10.3847/2041-8213/abbe26 |arxiv=2009.04771 |bibcode=2020ApJ...903L...5R |doi-access=free}}</ref>
Using eccentric waveforms based on [[numerical relativity]], Gayathri et al. 2020 found a best fit with <math>e_{\rm 10 Hz}=0.67</math> and source masses {{solar mass|{{val|102|7|11}}}} for both merging black holes. 
<ref name="Gayathri+2020">{{cite arXiv |author=Gayathri, V. |display-authors=et al. |title=GW190521 as a Highly Eccentric Black Hole Merger |year=2020 |class=astro-ph.HE |eprint=2009.05461 }}</ref>

==See also==
* [[Gravitational-wave astronomy]]
* [[List of gravitational wave observations]]
* [[Multi-messenger astronomy]]

==Notes==
{{reflist|group=note}}

==References==
{{reflist}}

==External links==
* {{cite web|url= https://www.ligo.org/detections/GW190521.php|title=GW190521|publisher=[[LIGO]]}}
* {{YouTube|JtpO5S1XhG8|Video (1:14:15): Webinar GW190521}} ([[LIGO Scientific Collaboration|LIGO]]; 3 September 2020).
* {{YouTube|zRmwtL6lvIM|Video (00:30): Simulation of Black-Hole Merger}} ([[Max Planck Institute for Gravitational Physics|AEI]]; 2 September 2020).
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