How a black hole collision let off an explosion of light

It sounds impossible, but scientists think they've found proof that when two black holes collide, they can give off a massive burst of light.

After detecting the gravitational waves - disturbances in the curvature of spacetime - of such a collision in 2019, astronomers then spotted a flare of light coming from the same region of distant space.

Both events happened where a supermassive black hole called J1249+3449 is known to reside. 

"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," explains KE Saavik Ford, an astrophysicist at the City University of New York (CUNY).

"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, organising the black holes so they can pair up."

While supermassive black holes are invisible, so strong not even light can escape, they are surrounded by a disk of spinning glowing gas. The first-ever photograph of a black hole, taken last year, showed a flaming orange ring around the black hole at the centre of Messier 87, 54 million light-years from Earth.

When the smaller black holes merge, the energy released causes the new entity to fire off in a random direction - in this case, right through the disk of gas.

"It is the reaction of the gas to this speeding bullet that creates a bright flare, visible with telescopes," said co-author astrophysicist Barry McKernan, also of CUNY.

The disturbance was first noted by astronomers at the National Science Foundation's Laser Interferometer Gravitational-wave Observatory in May 2019. Astronomers at a separate institute - Caltech's Zwicky Transient Facility (ZTF) in San Diego - had a look at their recording of the event, and that's when evidence for the flare was spotted.

"The flare occurred on the right timescale, and in the right location, to be coincident with the gravitational-wave event," said study lead author Matthew Graham of ZTF.

"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."

They looked at 15 years of data from J1249+3449, and found nothing else like it however. The new object is expected to pass through the gas disk again in the next few years. 

"The reason looking for flares like this is so important is that it helps enormously with astrophysics and cosmology questions," said co-author Mansi Kasliwal, assistant professor of astronomy at Caltech. 

"If we can do this again and detect light from the mergers of other black holes, then we can nail down the homes of these black holes and learn more about their origins."

The research was published in journal Physical Review Letters.