GW190521

GW190521 (initially S190521g)^[5] was a gravitational wave signal resulting from the merger of two black holes. 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.^[2][6] The event was observed by the LIGO and Virgo detectors on 21 May 2019 at 03:02:29 UTC,^[7] and published on 2 September 2020.^[4][5][8] The event had a Luminosity distance of 17 billion light years away from Earth,^{[note 1][5][9]} within a 765 deg² area^{[note 2][10]} towards Coma Berenices, Canes Venatici, or Phoenix.^{[1][2][6][11]}

GW190521

The GW event GW190521 observed by the LIGO Hanford (left), LIGO Livingston (middle), and Virgo (right) detectors
Date	21 May 2019
Instrument	LIGO, Virgo^[1]^[2]
Right ascension	12^h 49^m 42.3^s^[3]
Declination	−34° 49′ 29″^[3]
Epoch	J2000.0
Distance	5,300 megaparsecs (17,000 Mly)^[4]
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At 85 and 66 solar masses (M_☉) respectively, the two black holes comprising this merger are the largest progenitor masses observed to date.^[12] 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.^[4][5][8]

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 65 M_☉, leaving a black hole mass gap above 65 M_☉. The 85+21
−14 M_☉^{[note 3]} and 142+28
−16 M_☉ black holes observed in GW190521 are conclusively in the mass gap, indicating that it can be populated by the mergers of smaller black holes.^[4]

Only indirect evidence for intermediate mass black holes, those with between 100 and 100,000 solar masses, had been observed earlier, and it was unclear how they had formed.^[13] 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.^[8]

According to discovery team member 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."^[9]

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 source related to the merger of two black holes.^{[2][3][6][14]} 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 200 km/s (120 mi/s) through the disk, according to the astronomers.^[15] If this explanation is correct, the flare should repeat after about 1.6 years^[3] when the intermediate mass black hole again encounters the accretion disk.^[15] As of 2023, the status of the connection between these two events is unconfirmed.^[16]

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."^[15]

Possible eccentricity

While the original LIGO/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 ${\displaystyle e_{\rm {10Hz}}\geq 0.1}$  than a spin-precessing quasi-circular model.^[17] Using eccentric waveforms based on numerical relativity, Gayathri et al. 2020 found a best fit with ${\displaystyle e_{\rm {10Hz}}=0.67}$  and source masses 102+7
−11 M_☉ for both merging black holes. ^[18]

See also

• Gravitational-wave astronomy
• List of gravitational wave observations
• Multi-messenger astronomy

Notes

1. "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."^[9]
2. The relatively large and distant area of the sky within which it is claimed to be possible to localize the source.
3. This notation is used to state asymmetric uncertainty.

References

1. "Superevent info - S190521g". LIGO. 21 May 2019. from the original on 28 June 2020. Retrieved 25 June 2020.
2. Cofield, Calla (25 June 2020). "Black Hole Collision May Have Exploded With Light". NASA. from the original on 28 June 2020. Retrieved 25 June 2020.
3. Graham, M.J.; et al. (2020). "Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational-Wave Event S190521g" (PDF). Physical Review Letters. 124 (25): 251102. arXiv:2006.14122. Bibcode:2020PhRvL.124y1102G. doi:10.1103/PhysRevLett.124.251102. PMID 32639755. S2CID 220055995. (PDF) from the original on 11 July 2020. Retrieved 5 September 2020.
4. Abbott, R.; et al. (2 September 2020). "Properties and Astrophysical Implications of the 150 M ⊙ Binary Black Hole Merger GW190521". The Astrophysical Journal. 900 (1): L13. arXiv:2009.01190. Bibcode:2020ApJ...900L..13A. doi:10.3847/2041-8213/aba493.
5. Abbott, R.; et al. (2 September 2020). "GW190521: A Binary Black Hole Merger with a Total Mass of 150 M ⊙". Physical Review Letters. 125 (10): 101102. arXiv:2009.01075. Bibcode:2020PhRvL.125j1102A. doi:10.1103/PhysRevLett.125.101102. PMID 32955328.
6. Overbye, Dennis (25 June 2020). "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". The New York Times. from the original on 25 June 2020. Retrieved 25 June 2020.
7. "GW trigger S190521g ('GW 190521')". University of Leicester. 2020. from the original on 28 June 2020. Retrieved 26 June 2020.
8. Martin (2 September 2020). "GW190521: The Most Massive Black Hole collision Observed To Date" (PDF). LIGO Scientific Collaboration. (PDF) from the original on 4 September 2020. Retrieved 2 September 2020.
9. Overbye, Dennis (3 September 2020). "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". The New York Times. from the original on 2 September 2020. Retrieved 4 September 2020.
10. Mo, Geoffrey (21 May 2020). "GCN Circular - Number: 24640 - LIGO/Virgo S190521g: Updated sky localization". NASA. from the original on 27 June 2020. Retrieved 27 June 2020.
11. Graduate Center, CUNY (25 June 2020). "Black hole collision may have exploded with light". Phys.org. from the original on 26 June 2020. Retrieved 26 June 2020.
12. Siegel, Ethan (3 September 2020). "LIGO's Biggest Mass Merger Ever Foretells A Black Hole Revolution". Forbes. from the original on 4 September 2020. Retrieved 5 September 2020.
13. Miller, M. Coleman; Colbert, E. J. M. (2004). "Intermediate-Mass Black Holes". International Journal of Modern Physics D. 13 (1): 1. arXiv:astro-ph/0308402. Bibcode:2004IJMPD..13....1M. doi:10.1142/S0218271804004426. S2CID 118959484.
14. "Black hole collision may have exploded with light". ScienceDaily. 26 June 2020. from the original on 26 June 2020. Retrieved 26 June 2020.
15. Carpineti, Alfredo (25 June 2020). "Astronomers May Have Observed The First Flare From A Black Hole Collision". IFLScience. from the original on 29 June 2020. Retrieved 26 June 2020.
16. Cui, Zhe; Li, Xiang-Dong (16 June 2023). "On the formation of GW190521-like binary black hole merger systems". Monthly Notices of the Royal Astronomical Society. 523 (4): 5565–5582. arXiv:2306.08441. doi:10.1093/mnras/stad1800. ISSN 0035-8711.
17. Romero-Shaw, I.; et al. (23 October 2020). "GW190521: Orbital Eccentricity and Signatures of Dynamical Formation in a Binary Black Hole Merger Signal". The Astrophysical Journal. 903 (1): 5. arXiv:2009.04771. Bibcode:2020ApJ...903L...5R. doi:10.3847/2041-8213/abbe26.
18. Gayathri, V.; et al. (2020). "GW190521 as a Highly Eccentric Black Hole Merger". arXiv:2009.05461 [astro-ph.HE].

External links

• "GW190521". LIGO.
• Video (1:14:15): Webinar GW190521 on YouTube (LIGO; 3 September 2020).
• Video (00:30): Simulation of Black-Hole Merger on YouTube (AEI; 2 September 2020).