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.
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.