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Destroyed Star Rains onto Black Hole, Winds Blow it Back

For Release: October 21, 2015

CXC

SgrA*
Credit: Spectrum: NASA/CXC/U.Michigan/J.Miller et al.; Illustration: NASA/CXC/M.Weiss
Press Image and Caption

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 an extraordinary 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 results in tidal forces that can rip the star apart. In these events, called "tidal disruptions," some of the stellar debris is flung outward at high speeds, while the rest falls toward the 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 Burst Explorer, and ESA/NASA's XMM-Newton collected different pieces of this astronomical puzzle in a tidal disruption event called ASASSN-14li, originally discovered in an optical search by the All-Sky Automated Survey for Supernovae (ASAS-SN) in November 2014.

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 force pulls 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 black hole's event horizon, the point beyond which no light can escape.

Gas often falls toward 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 tracking 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."

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