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It's been about five years since scientists first recorded evidence of a pair of black holes smashing and merging together. That evidence took the form of gravitational waves, the rippling of space-time itself.

Those waves have taught us a lot, but something has also been missing: unlike most things in astronomy, we haven't been able to see any of these mergers with light. That kind of makes sense, since black holes are, you know, completely black. But for a while, scientists have suspected that there is a way these collisions could light up. 

And last week in Physical Review Letters, astronomers reported that they may have finally seen it happen. By definition, black holes don't give off light of their own, but we can still see many of them because they tend to heat up nearby gas and dust. And as that material gets hotter, it glows. The problem is, this hasn't been relevant for the mergers we've detected among pairs of black holes because those mergers have generally happened way out in the middle of nowhere where there's not much dust or gas to be found.

But hypothetically, if a collision happened in a denser area of space, we might see something. Like, say, at the center of a galaxy. In the hearts of some galaxies, supermassive black holes are actively gobbling up the matter swirling around them in an accretion disk. Astronomers call these areas active galactic nuclei, or AGNs. 

But turns out AGNs can have more than just one black hole: they can also be home to a pair of much smaller black holes that might one day emerge together. Theoretically, when that happens, the new black hole from the merger gets a huge kick. Its path around the center of the galaxy shifts and it plows across the accretion disk, causing the gas to release an extra burst of light that fades over the following weeks. 

In last week's study, a team of astronomers was hunting for events like this. They were trying to match gravitational wave events with flares of light coming from the same point in the sky within a few weeks of one another. And in some data from last year, it seems like they found one.

In May 2019, two key observatories found a gravitational wave candidate likely coming from the AGN inside a distant galaxy. And based on the waves, it seems to have come from a pair of colliding black holes. Meanwhile, the ZTF sky survey recorded a flare of visible light coming from that same galaxy about five weeks later. 

If these events are related, this would confirm our hypothesis and give significant insight into the physics of black hole mergers. Like, it could teach us more about exactly where the black holes came from and how they got there.

But it's important to point out that the flare's origins still aren't 100% certain. The team has worked to rule out a supernova, a typical flare that happens when a black hole eats a star, and just normal activity from the supermassive black hole, but there's still some uncertainty.

And technically, the gravitational wave data has yet to be confirmed as an actual merger. Still, there's some good news here. If the models are correct, the new black hole will cause another flare in the accretion disk in a few years. So we just need to keep our eyes and our telescopes open.