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Black holes don't have to be active to warp space and time

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Even dormant black holes warp space and time, scientists now say. The discovery came from a supermassive black hole that’s a few million Solar masses and is located at the center of a galaxy 3.8 billion light years away.

Black holes are so incredibly dense that nothing — not even light — can escape their gravitational pull. So we can’t really observe them directly. To study them, scientists detect the X-rays emitted by the nearby matter that accelerates and heats up as it gets sucked in by the black hole.

This dormant black hole tore a star apart 3.8 billion light years away

But this star-eater is a special kind of black hole: a dormant black hole, basically a sleeping giant that doesn’t devour much gas. About 90 percent of all supermassive black holes in the Universe are dormant, but we know little about them because they’re hard to study. They don’t suck in as much matter, so they’re harder to see. The dormant black hole in this case pulled a star apart, giving scientists the opportunity to see it with X-rays. And the observation also let scientists observe gravity effects from a dormant black hole for the first time, according to a study published in Nature.

Scientists know more about active black holes, which feed on lots of X-ray-emitting material. They also have a disc of gas and stellar debris surrounding them, a so-called accretion disc, which helps scientists study the gravity effects of the black hole, as well as its spin. But active black holes make up only 10 percent of all the supermassive black holes in the Universe, so studying the much more common dormant holes is key to understanding the overall nature of black holes.

"We are seeing these strong gravity effects from a dormant black hole for the first time," says study co-author Erin Kara, a post-doc fellow at the University of Maryland, College Park, and at NASA Goddard Space Flight Center. "And that’s important because there has been this effort to measure black holes’ spin in all black holes, but usually you can do it only in these active black holes."

A stellar debris field

The researchers were able to study this particular dormant black hole thanks to the X-rays emitted when a star was torn apart by its huge gravitational pull — an event called "tidal disruption." The disruption event was first detected by NASA’s Swift Burst Alert Telescope on March 28th, 2011. When the star was ripped apart, stellar debris fell around the black hole, forming a temporary, gaseous disc that heated up and radiated light. "This is one way we can see these dormant black holes," says Kara.

By studying the light emitted, the researchers were able to see how dormant black holes warp space and time, gobbling up hot gas to feed and grow in the process. They also figured out that the disc surrounding the black hole was puffy and not as thin as the accretion disc of active black holes.

What's the spin of a dormant black hole?

"This is helping us understand how material can fall so rapidly into a supermassive black hole," Kara says. "This is something that is important not just for understanding tidal disruption events, but for understanding how black holes gain mass in general, how they feed in general."

Eventually, the researchers want to calculate how fast this particular dormant black hole is rotating. Because scientists estimate that 90 percent of all black holes in the Universe are similar to the one described in today’s study, they think this dormant black hole could help them understand what’s the spin of most of the black holes in the Universe.


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