Destroyed star rains onto black hole, winds blow it back

Posted by ap507 at Oct 22, 2015 10:10 AM |
'Tidal disruptions’ from black holes observed by international team including Leicester astronomers

Issued by University of Leicester Press Office on 22 October 2015

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New details about what happens when a black hole tears apart a star have been gathered by a trio of orbiting X-ray telescopes giving scientists, including University of Leicester astronomers, an excellent opportunity to understand the extreme environment around a black hole.

When a star comes too close to a black hole, the intense gravity of the black hole can rip the star apart. In these events, called “tidal disruptions,” some of the stellar debris is flung outward at high speeds, while the remainder falls into black hole. This causes a distinct X-ray flare that can last for a few years.

NASA’s Chandra X-ray Observatory, Swift Gamma Ray Explorer, and ESA’s XMM-Newton were used to put together different pieces of this astronomical puzzle in a tidal disruption event called ASASSN-14li. The event occurred near a supermassive black hole estimated to weigh a few million times the mass of the Sun. The black hole is located in the center of PGC 043234, a galaxy that lies about 290 million light years from Earth. This makes this event the closest tidal disruption discovered in a decade.

“We have seen evidence for a handful of tidal disruptions over the years and have developed a lot of ideas of what goes on,” said Jon Miller of the University of Michigan in Ann Arbor, who led the study that is described in a paper published in the latest issue of Nature. “This one is the best chance we have had so far to really understand what happens when a black hole shreds a star.”

After the star is destroyed, the black hole’s strong gravitational forces pull most of the remains of the star toward it. This infalling debris is heated to millions of degrees and generates a huge amount of X-ray light. Soon after this surge of X-rays, the amount of light decreases as the material falls beyond the event horizon (that is, the point of no return) of the black hole.

Paul O’Brien from the University of Leicester, a member of the Swift team and a co-author on the paper said: “Black holes only emit light when they interact with material that comes close to them, in this case an entire star. Unfortunately for this star, it got too close and was completely destroyed allowing us to witness the intense power of a black hole”.

Gas often falls towards black holes by spiraling inward in a disk. But how this process starts has remained a mystery. In ASASSN-14li, astronomers were able to witness the formation of such a disk by looking at the X-ray light at different wavelengths (known as the "X-ray spectrum") and how that changed over time.

The researchers determined that the X-rays being produced come from material that is either very close to or is actually in the smallest possible stable orbit around the black hole.

“The black hole tears the star apart and starts swallowing material really quickly, but that’s not the end of the story,” said co-author Jelle Kaastra of the Institute for Space Research in the Netherlands. “The black hole can’t keep up that pace so it expels some of the material outwards.”

The X-ray data reveal the presence of a wind moving away from the black hole. The wind is not moving fast enough to escape the black hole’s gravitational grasp. The relatively low speed for the wind may be explained by gas from the disrupted star that’s following an elliptical orbit in a newly formed disk around the black hole.

“These results support some of our newest ideas for the structure and evolution of tidal disruption events,” said Cole Miller, a co-author from the University of Maryland in College Park. “In the future, tidal disruptions can provide us with laboratories to study the effects of extreme gravity.”

Astronomers are hoping to find more events like ASASSN-14li, which they can use to continue to test theoretical models about how black holes affect their environments and anything that might wander too close.

“It looks like this black hole is a very similar size to the one that lurks in the centre of our own Milky Way galaxy, although thankfully the Sun’s orbit stays far away from it, so there’s no danger of us suffering the same nightmare scenario”, commented Professor Nial Tanvir, another Leicester researcher who took part in the work.

These results appear in the October 22nd issue of the journal Nature. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations. Swift is managed by NASA's Goddard Space Flight Center. It was built and is being operated in collaboration with Penn State, the Los Alamos National Laboratory, and General Dynamics in the U.S.; the University of Leicester and Mullard Space Sciences Laboratory in the United Kingdom; and Brera Observatory and the Italian Space Agency in Italy. XMM-Newton is operated by the European Space Agency.

Ends

Notes to editors:

The University of Leicester researchers are currently travelling and interviews will not be possible. Professor Paul O’Brien will have limited access to e-mails, please direct enquiries to pto2@leicester.ac.uk.

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