Scientists have measured a fundamental property of a supermassive black hole – how fast it rotates – by measuring a star hitting it.
It can be difficult to measure black holes unless they actually do something, such as when they hatch or vomit jets of matter. But the scientists behind the new result were able to measure the mass and rotation of a fairly massive black hole, demonstrating that these brief star-eating events, called tidal-breaking events, could offer another way to understand black holes.
"There have already been measurements of black hole spins that are actively accumulating," or acquiring more matter under the influence of gravity, said study author Dheeraj Pasham, Einstein Postdoctoral Fellow at the MIT Kavli Institute, at Gizmodo. "This measure is different, in the sense that we were able to measure the turning of a black hole that was inactive," at least until the event of tidal rupture occurs.
An automatic sky survey called the All-Sky Automatic Survey for SuperNovae, or ASASSN, spotted the flash on November 22, 2014. The flash, called ASASSN-14li, looked like the typical black hole gravity crusher of a star-to-bits, happening near the center of a host galaxy. Scientists immediately looked for "quasi-periodic oscillations," repeating regularly, but changing the x-ray patterns that vary in their power and are believed to originate from very close to the black hole. They found what they were looking for in data from two x-ray space telescopes.
Scientists used these x-ray emissions to infer the mass and rotation of the black hole. And in that case, the researchers estimated that the mass was between a few hundred thousand and 10 million times the mass of the Sun, and the spin was incredible at 50% of the speed of light, according to an MIT statement based on the findings of the paper, published yesterday in the journal Science.
Pashnam cautioned that these values are still based on a model, and there may be several ways of interpreting the same data. And this is just a data point, so you should not think too much about the spin and mass values yet. Instead, the paper demonstrates a new and important way of measuring properties of all black holes, not just a special set of them.
A researcher not involved in the study, James Guillochon, ITC Fellow of the Harvard-Smithsonian Center for Astrophysics, agreed. "Their results seem consistent with other works (specifically [ASASSN-14li], but also work on the spin of the supermassive black hole), "he told Gizmodo. "The result suggests that tidal-breaking events should be regularly monitored by X-ray telescopes to maximize our knowledge of their properties."
This is indeed the goal, Pasham told Gizmodo. He explained that these tidal-breaking events occur every thousand to ten thousand years per galaxy, so they hope they can one day measure several hundred of these events each year to get a general picture of black hole properties.
And no need to worry: our own star will not be one of those that will hit a supermassive black hole anytime soon.