GW 190521

Astronomers had long suspected an in-between class called intermediate-mass black holes, weighing 100 to more than 10,000 solar masses. While a handful of candidates have been identified with indirect evidence, the most convincing example to date came on May 21, 2019, when the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO), located in Livingston, Louisiana, and Hanford, Washington, detected gravitational waves from a merger of two stellar-mass black holes. This event, dubbed GW190521, resulted in a black hole weighing 142 Suns.

GW190521 (initially S190521g) 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. The event was observed by the LIGO and Virgo detectors on 21 May 2019 at 03:02:29 UTC, and published on 2 September 2020. The event was 17 billion light years away from Earth, within a 765 deg² area towards Coma Berenices, Canes Venatici, or Phoenix.

At 85 and 66 solar masses (M_☉) respectively, the two black holes comprising this merger are the largest progenitor masses observed to date. 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 as energy in the form of gravitational waves.