GW190521 (or GW190521g; initially, S190521g)[5] is 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 was 17 billion light years away,[n 1][5][9] within a 765 deg2 area[n 2][10] towards Coma Berenices, Canes Venatici, or Phoenix.[1][2][6][11]

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 as energy in the form of gravitational waves.[4][5][8]

GW190521 is a significant discovery due to the masses of the resulting large black hole, and one or both of the smaller constituent black holes. Stellar evolution theory predicts that stars cannot produce black holes of more than about 65 M☉, so there should be a black hole mass gap above that value. The 85+21−14 M☉ 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 "a hierarchical merger, in which the two progenitor black holes themselves may have formed from the merging of two smaller black holes, before migrating together and eventually merging."[8]