When scientists spotted this pair of black holes, it was a rare chance to observe black holes in the process of colliding. Soon, however, as they looked closer, scientists were consumed with a brand new question: Uh, hey, what’s that blinking light?

The light isn’t coming from the pair of colliding black holes (named PG 1302-102) 3.5 billion light years away from us, it’s coming from the turbulence around them. What that doesn’t explain, though, is why the light “flashes” rhythmically—regularly brightening, then dimming. So researchers Daniel D’Orazio, Zoltan Haiman, and David Schiminovich at Columbia University built a simulation of the pair and have now come up with an explanation for just what we’re seeing.

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It’s the orbit of the black holes.

Black hole collision is not a neat or particularly equitable process—one is stronger, bigger, and more powerful than the other. What that all translates too is a black hole that’s kicking-up a whole lot more turbulence in its wake than its binary partner. Curiously, though, this is not the brighter black hole, instead its the smaller one that’s glowing stronger.

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Why? It’s because the smaller one is less able to kick around the interstellar dust than its larger companion. That means that more gas ends up drifting near it than the larger one, and its the gas that gives off that bright glow.

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In an effect that the researchers compared to the Doppler effect, when the brighter black hole orbits closer to us, we get more of its light, as it moves further it dims. That orbit also explains the cyclical nature of the signal, which appeared to dim and brighten on a regular schedule.

Their full paper is out today in Nature.

Images: Columbia University

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