In spite of their dark reputations, two black holes may have set off a cosmic light show.

Subtle gravitational rumbles from a collision of two black holes may have been accompanied by a flare of light about a month later, physicists report June 25 in Physical Review Letters. It’s a surprising conclusion given black holes’ propensity to swallow up light and matter. “The normal expectation has been they just merge and all you would detect is gravitational waves,” says astrophysicist Matthew Graham of Caltech.

But scientists, not ones to rest on assumptions, wanted to check if that expectation was right. To look for a flare, Graham and colleagues combed through data from the Zwicky Transient Facility at the Palomar Observatory in California, which repeatedly images the sky, searching for short-lived changes called transients. They found that about 34 days after the gravitational waves were detected in May 2019, a blaze of light appeared in the vicinity of sky that the gravitational waves had pinpointed. This outburst was associated with a known quasar, a glowing celestial object made up of a disk of gas surrounding a supermassive black hole (SN: 3/29/16). The gargantuan black hole in question boasts a mass 100 million times that of the sun.

The researchers suggest that the flare could have been produced if two smaller black holes met up in the vicinity of that supermassive black hole, coalescing within the swirling gas disk. That unification could have flung the resulting merged black hole through the disk, creating a shock wave that heated the gas, producing a temporary burst of light.

If the theory is correct, the researchers calculate that the two smaller black holes had a total mass about 100 times that of the sun. After the merger, the resulting single black hole would have plowed through the gas at a speed of about 700,000 kilometers per hour before leaving the disk. In the future, that black hole should swing back thanks to the gravitational pull of the disk, causing another flare in late 2020 or early 2021. Spotting that predicted flare would help confirm the explanation.

The connection between the flare and the gravitational waves isn’t certain, says astrophysicist Daniel Holz of the University of Chicago, a member of the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, one of two observatories that detected the gravitational waves (SN: 6/23/20).

“The problem is that the sky is incredibly dynamic and lively. There are stars exploding and black holes burping and stars being ripped apart,” Holz says. So the flare may just be a strange coincidence. But, he says, if it is real, “it would provide a whole new window on how black holes are made and live and die.”

LIGO, based in the United States, and the Advanced Virgo detector in Italy, have spotted many pairs of merging black holes, but scientists don’t know how the black holes find one another (SN: 5/2/19). Previous work has focused on black holes meeting up within a cluster of stars, for example (SN: 6/19/16). The possibility that they might pair up within an accretion disk is a newer hypothesis.  “It started out as kind of a fringe idea,” says astrophysicist Jillian Bellovary of Queensborough Community College in New York City, who was not involved in the research. But, “it has definitely been gaining traction in the community.”

If it’s correct, the result could mean that future gravitational wave detections could help unravel the messy, poorly understood physics that goes on within such disks, says astrophysicist Richard O’Shaughnessy of Rochester Institute of Technology in New York. That in turn could help scientists understand how galaxies with quasars evolve as energy churned up by a supermassive black hole feeds back into the galaxy. “It’s a bold interpretation opening a new connection between gravitational waves and the night sky.”