A supermassive black hole lit up a collision of two smaller black holes

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Astronomers from CalTech have reported that they've observed a collision between two black holes. Normally such an event is invisible, but this time a more massive black hole sitting nearby helped illuminate the other two as they collided. If confirmed, the findings, published in Physical Review Letters, would be the first optical observations ever made of a black hole merger.

What happened: First detected in May 2019 and dubbed S190521g, the merger happened about 4 billion light-years away, within the vicinity of a supermassive black hole called J1249+3449. This object is 100 million times more massive than the sun, with a diameter roughly the size of Earth’s orbit around the sun. 

The two smaller black holes, each one about 150 times more massive than the sun, stumbled into the supermassive's accretion disk: a swirl of stars, gas, and dust that is slowly being sucked toward its event horizon, beyond which not even light can escape. 

When the two smaller black holes collided, the force sent the merged object hurtling out of the accretion disk at about 700,000 kilometers per hour, the researchers calculate. As it sped into space, the merged black hole lit up the surrounding gas in the disk, producing light that was a trillion times brighter than the sun. 

How astronomers linked the merger with the supermassive black hole: S190521g, like many other black hole mergers, produced ripples in space-time that were detected on Earth by LIGO, a gravitational wave observatory. When those observations came in, an automated alert was sent out to the world’s telescopes to see if they could optically observe any events in the night sky that might coincide with the merger. About 34 days later, the Zwicky Transient Facility in California spotted the light produced by the merger event, and scientists followed a fiery trail back to J124942.3+344929. 

The implications: Black holes are not supposed to be visible, but these new findings suggest we can actually visualize these objects by observing the surrounding matter they light up. It's not all that different from how the Event Horizon Telescope snapped the now-famous image of a supermassive black hole, which was released last year. That image is not exactly the black hole itself, but rather the glowing gas and dust bordering its event horizon.

Establishing a way to closely observe black hole mergers in accretion disks might allow us to answer questions about how matter interacts with these objects, and whether a merger can be predicted before it happens. The Caltech team believes it’s possible these black holes were the result of a long chain of previous mergers—and S190521g was simply the latest.

What’s next: The findings still have to be confirmed. Astronomers will be doing a more detailed analysis of S190521g to see if the merger and the flare associated with J124942.3+344929 really are connected.  

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