Black holes aren’t supposed to make flashes of light. It’s right there in the name: black holes. Even when they slam into each, the massive objects are supposed to be invisible to astronomers’ traditional instruments. But when scientists detected a black holes collision last year, they also spotted a weird flash from the crash. On May 21, 2019, Earth’s gravitational wave detectors caught the signal of a pair of massive objects colliding, sending ripples cascading through spacetime. Later, an observatory called the Zwicky Transient Facility (ZTF) caught a blast of light. As scientists looked at the two signals, they realized both came from the same patch of sky, and researchers started wondering whether they had spotted the rare visible black hole collision.
Here’s what scientists think happened in this strange case. The two black holes that merged were locked in the disk surrounding a quasar, a supermassive black hole that shoots out blasts of energy. “This supermassive black hole was burbling along for years before this more abrupt flare,” Matthew Graham, an astronomer at Caltech and the project scientist for ZTF, said in a university statement. That in and of itself isn’t so strange, according to his colleague. “Supermassive black holes like this one have flares all the time,” co-author Mansi Kasliwal, an astronomer at Caltech, said in the statement. “They are not quiet objects, but the timing, size and location of this flare was spectacular.”
Scientists suspect, based on the pairing of gravitational waves and light, that the flare sprang from two small black holes merging within the accretion disk of the supermassive black hole. The supermassive black hole’s incredibly strong gravity affects the smaller stuff in the disk, even other black holes. The flash of light doesn’t come from the merger itself, the scientists think. Instead, the force of the merger sends the now-a-little-larger black hole flying off, through the gas surrounding it in the supermassive black hole’s accretion disk. The gas, in turn, produces the flare after a delay of days or weeks, the theory goes according to the statement. In the case of this event, scientists detected the flare about 34 days after the gravitational wave signal.
That’s not a guarantee that this explanation fits what happened, the researchers said. “The flare occurred on the right timescale, and in the right location, to be coincident with the gravitational-wave event,” Graham said. “We conclude that the flare is likely the result of a black hole merger, but we cannot completely rule out other possibilities.”