black hole spin speed devour star

Dinner Time

Apparently, you can learn a lot by watching a black hole eat.

In 2014, astronomers spied a supermassive black hole nearly 300 million light-years away gobbling a star, an event known as a tidal disruption flare or TDE. Given the rarity of such a spotting, they’ve kept a close eye on it ever since.

The hope was that it would teach them something new about how black holes feed. But it ended up teaching a team at the Massachusetts Institute of Technology something else: a new way to measure how fast a black hole is spinning.

Strange Signal

The MIT team describes its discovery in a paper published in the journal Science on Wednesday.

While looking at TDE data collected by three different observatories — XMM-Newton, Chandra, and Swift — the MIT team noticed a strong burst of X-ray radiation pulsing every 131 seconds for at least 450 days straight.

“At first I didn’t believe it because the signal was so strong,” MIT researcher Dheeraj Pasham said in a news release. “But we saw it in all three telescopes. So in the end, the signal was real.”

Next, the team set out to determine the cause of the X-ray signal. Based on their observations, they believe a bit of the star the black hole was eating remained outside its event horizon where it came in contact with a white dwarf that was also circling the black hole.

Alone, this dwarf would be invisible to telescopes. But combined with the stellar material, it produced the kind of X-ray radiation the MIT team detected. Each pulse of the signal was the pair completing another orbit around the black hole.


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