Black hole tears star apart

Observations of a black hole tearing apart a nearby star are giving astronomers a rare insight into the process, known as a 'tidal disruption'.

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In this artist’s depiction, a disc of stellar debris is seen around the black hole in the upper left, with a tail of debris extending to the right. In the inset box is a graph showing evidence for dips in X-ray intensity over a narrow range of wavelengths.
Credit: NASA/CXC/M. Weiss

New observations of a black hole have given astronomers the closest view yet of the X-ray flares that occur when a black hole’s gravity tears apart a nearby star.

The supermassive black hole is thought to weigh a few million times the mass of the Sun and is found at the centre of galaxy PGC 043234, 290 million lightyears from Earth.

This is the closest observed example of the process and is enabling scientists to gain further understanding of the environment around black holes.

When a star gets too close to a black hole, the black hole’s gravity can tear the star apart in what is called a ‘tidal disruption’. This event causes some of the stellar material to be projected out into space, while some falls into the black hole, causing a massive X-ray flare.

NASA’s Chandra X-ray Observatory, Swift Gamma-ray Burst Explorer and ESA/NASA's XMM-Newton were all involved in collecting data of the event, called ASASSN-14li. It was originally discovered in an optical search by the All-Sky Automated Survey for Supernovae (ASAS-SN) in November 2014.

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

Once the star is completely destroyed, some of the stellar debris falls into the black hole, heating to millions of degrees and projecting a massive amount of X-ray light. This light then disappears over time as the material falls beyond the black hole’s event horizon: the point at which no light can escape.

This latest observation has revealed new details as to how the process works. Gas usually falls toward black holes by spiralling inward in a disc, but how exactly this occurs has been a mystery. In the case of ASASSN-14li, astronomers were able to witness the formation of the disc by observing the X-ray light at different wavelengths and how that changed over time.

The team concluded that the X-rays 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."

Further revelations from the data include the suggestion that a wind is moving away from the black hole carrying the stellar gas with it, but not fast enough to escape its gravitational pull. An explanation for the low speed of the wind is that gas from the star is moving in an elliptical orbit around the black hole, but at the greatest distance, where it is traveling the slowest.

More events like ASASSN-14li will help astronomers create more theoretical models about how black holes affect their environments.


 
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