Most black hole mergers occur in darkness. Literally, the events don’t emit any light.
At least not generally. Several years ago, theorists predicted that if a black hole merger occurred in the heart of a galaxy, the merger of the two black holes, under the gravitational influence of a supermassive black hole, could cause the resulting merged black hole to fly out of the heart of the galaxy and into the surrounding material. This rapid exit could cause the material being plowed through to light up. And now an example of this kind of a black hole making things light up seems to have been found.
In May of 2019, a gravitational wave detection was made in the direction of the quasar 1249+3449. I haven’t been able to find the ratio of the masses that merged, but the resulting new black hole is one hundred solar masses. Several days after the merger was detected, the Zwicky Transient Facility detected a flare-up that didn’t match a black hole. In a new paper in Physical Review Letters, a team lead by Matthew Graham describes how this one-two punch of observations matched the earlier predictions.
This is all described beautifully by co-author K. E. Saavik Ford of the City University of New York, who explains: At the center of most galaxies lurks a supermassive black hole. It’s surrounded by a swarm of stars and dead stars, including black holes. These objects swarm like angry bees around the monstrous queen bee at the center. They can briefly find gravitational partners and pair up but usually lose their partners quickly to the mad dance. But in a supermassive black hole’s disk, the flowing gas converts the mosh pit of the swarm to a classical minuet, organizing the black holes so they can pair up.
A gravitational wave showed us the pair up, and then optical telescopes showed us the pair on the run.
So the theorists got it right, and now we know we need to be wary of runaway black holes because really, they are a thing.
“A Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational Wave Event GW190521g,” Matthew Graham et al., 2020 June 25, Physical Review Letters