It might just be possible to see a light flash too when black holes merge

Black hole merger events are some of the most energetic, fearsomely energetic events in all the cosmos. When black holes merge, they’re entirely invisible, the only evidence of the cataclysm some faint whisper of gravitational waves. Until now.

LIGO, the gravitational-wave detector extraordinaire, has been spitting black hole mergers since 2015, racking up a few mergers every year. And the more that the date pour in, the more curious events we spot.

Most black hole mergers are completely silent. Two bottomless pits of endless gravity find each other in the void between the stars, circle each other for eons in silence, and finally coalesce together, unleashing a fury of gravitational waves, ripples in the fabric of spacetime itself.

Enough energy is carried off by the gravitational waves in a typical merger event that if you were to convert that energy into mass (via Einstein’s simple E=mc^2 formula), you would have enough raw material to manufacture a dozen or so suns.

That’s a lot of energy, but gravity is stupidly weak. Unless you were very close to the merger event, the gravitational waves would wash through you without even budging your atoms. It takes the likes of LIGO, one of the Earth’s specially-designed detectors, to spot the subtle ripples in spacetime.

And those waves are usually the only evidence we ever have that a black hole merger took place.

A Flash in the Dark

But one recent gravitational wave event, called S190521g, strangely correlated with observations by the Zwicky Transient Facility (ZTF), a telescope at the Palomar Observatory designed to find and study quick flashes in the sky. The ZTF spotted a burst of electromagnetic radiation coming from the same region of space as the source of the gravitational wave signal.

The origin of both LIGO’s and ZTF’s observations appears to be the quasar called J1249+3449 (astronomers have really got to work on better names). The two observations didn’t just align in space, they happened close to each other in time, with the flare appearing a few days after the gravitational wave event.

So what’s going on? How did two merging black holes cause a bright flare of light? Nothing can escape the event horizon of a black hole, so it couldn’t have been coming from the merging pair itself.

The culprit appears to be the quasar itself, which hosts its own supermassive black hole in its heart. That black hole carries with it a massive, extended disk of material that spins and spins as it finds its way into the gaping maw of the black giant in the center.

Apparently, as best as astronomers can tell, two small black holes found themselves inside the accretion disk and eventually merged. That new, larger black hole got a big kick of gravity, boosting it through the disk, which triggered the eruption of the flare.

It’s an incredibly lucky coincidence, but as instruments like LIGO and ZTF continue to watch and listen to the night sky, we’re sure to find more, even stranger scenarios at play.

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