Scientists see light emerging from a black hole merger for the first ever time

Scientists think they have seen light coming out of a black hole merger for the first time.

When black holes swirl and crash into each other, they send ripples through spacetime that can be picked up on Earth as gravitational waves. But scientists had expected that such an event would not be seen as light, because black holes do not send out such waves.

Now, however, astronomers have spotted what they believe is a flare of light that was sent out from a pair of black holes as they merged into each other.

The event – referred to as S190521g – was first spotted by gravitational wave detectors last year. That functioned as an early warning, allowing researchers to know that something dramatic had happened deep in space.

At the same time, Caltech’s Zwicky Transient Facility (ZTF) was observing the sky as part of its surveys. It caught one flare that was later traced back to the same region as the gravitational wave event, allowing astronomers to finally link the two.

“This supermassive black hole was burbling along for years before this more abrupt flare,” said Matthew Graham, a research professor of astronomy at Caltech and the project scientist for ZTF. “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.”

The discovery is reported in a new paper, A Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational Wave Event GW190521g, which is newly published in Physical Review Letters.

Astronomers had previously theorised that it could be possible to see a flare emerge from such a black hole merger; the new observation marks the first time that such a phenomenon has been spotted.

Scientists suggest this could happen when two black holes are nestled in a disc that surrounds a much larger one.

“At the centre of most galaxies lurks a supermassive black hole. It’s surrounded by a swarm of stars and dead stars, including black holes,” said co-author KE Saavik Ford, of the City University of New York (CUNY) Graduate Centre, the Borough of Manhattan Community College (BMCC), and the American Museum of Natural History (AMNH).

“These objects swarm like angry bees around the monstrous queen bee at the centre. 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 disc, the flowing gas converts the mosh pit of the swarm to a classical minuet, organising the black holes so they can pair up,” she says.

The black holes merge, and the newly formed, much more massive black hole has a kick that sends it flying off and through the gas in the disc. As it speeds through that gas, it reacts, and it creates a bright flare that can be seen from Earth.

Researchers were only able to gather limited information about this black hole – by the time they came to carry out further observations, the flare had faded. In the future, they hope to gather more information about similar events, which could allow them to understand more about how the flare was formed.

With yet more light, and more information about that light, researchers hope they will be able to understand more about the regions of space in which these black holes are found, and how they can be formed.