Scientists Calculate Speed of Black Hole After Watching a Star Fall Into It

If you were a scientist and someone asked you to measure how fast a massive black hole was spinning, how would you do it? According to the experts, the properties of black holes can be hard to determine when they are just sitting out there undisturbed, but lunch time opens up the possibility to learn a lot about the mysterious regions of space. Using data from a a suite of space-based x-ray observatories, researchers at MIT observed a star falling into a supermassive black hole nearly 300 million light years away and were able to calculate its speed of rotation at around half the speed of light.

“There have already been measurements of spins from black holes that are actively accreting,” MIT Kavli Institute Einstein Postdoctoral Fellow (and first author of the study), Dheeraj Pasham told Gizmodo. “This measurement is different in the sense that we were able to measure the spin of a black hole that was dormant.” The star crashing into the black hole created what’s known as a tidal disruption, which created bursts of X-ray activity that were first identified in 2014 using the All-Sky Automated Survey for SuperNovae (ASASSN). While looking through datasets from other observatories including NASA’s Chandra and Swift and the ESA’s XMM-Newton space observatory, Pasham created a developed code to detect a pattern. What he found was an intense signal that pulsed every 131 seconds for 450 days. Using that information along with what they knew about the mass and size of the black hole, Pasham and his team were able to estimate its speed. 

“That’s not super fast — there are other black holes with spins estimated to be near 99 percent the speed of light,” Pasham said in a statement. “But this is the first time we’re able to use tidal disruption flares to constrain the spins of supermassive black holes.” While the new method is important, it is also not that useful because white dwarfs don’t fall into black holes 1 million times the mass of the Sun everyday. “We would’ve been extremely lucky to find such a system,” said Pasham,” but at least in terms of the properties of the system, this scenario seems to work.” The hope is that in the future, scientists can find similar events occurring at other black holes and use the method of observation to learn more about them.

Cover image: ESO/L. Calçada/M.Kornmesser CC BY 4.0

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