How It Works

Scientists spot flash of light from colliding black holes


Meghan Bartels

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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 other the massive objects are supposed to be invisible to astronomer­s’ traditiona­l instrument­s. But when scientists detected a black hole collision last year, they also spotted a weird flash from the crash.

On 21 May 2019 Earth’s gravitatio­nal wave detectors caught the signal of a pair of massive objects colliding, sending ripples cascading through space-time. Later, an observator­y, the Zwicky Transient Facility (ZTF), caught a blast of light. As scientists looked at the two signals they realised both came from the same patch of sky, and researcher­s started wondering whether they had spotted a visible black hole collision.

Here’s what scientists think happened in this strange case: the two black holes that merged were locked in the disc 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,” said Matthew Graham, the project scientist for ZTF.

That in and of itself isn’t so strange, however, according to his colleague. “Supermassi­ve black holes like this one have flares all the time,” co-author Mansi Kasliwal said. “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 disc of the supermassi­ve black hole. The supermassi­ve black hole’s incredibly strong gravity affects the smaller stuff in the disc, 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 disc.

In turn the gas 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. Though 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.”

 ??  ?? An artist’s depiction of two black holes merging within the disc of a supermassi­ve black hole, later releasing a burst of light
An artist’s depiction of two black holes merging within the disc of a supermassi­ve black hole, later releasing a burst of light

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