Mysterious behavior of a black hole: swallowed a star and spit it out 3 years later

Astronomers have discovered a black hole mysteriously spewing out pieces of an engulfed star a few years after it was consumed.

The event, which scientists have classified as AT2018hyz, began in 2018 when astronomers saw a black hole grab an unfortunate star with its strong gravitational pull before tearing it apart.

Then, three years later, in 2021, a radio telescope in New Mexico picked up a signal indicating unusual activity – a black hole began to erupt a star at half the speed of light.

Black holes have previously been seen to devour stars before regurgitating them, but so far, the ejection has only occurred during the “eating”. The researchers used four ground-based observatories around the world and two space-based observatories to record the event.

The scientists’ work was published in The Astrophysical Journal.

“It took us by surprise — no one had ever seen anything like it before,” said the lead author, an astrophysicist at the Harvard and Smithsonian Center for Astrophysics, Yvette Sendes.

The absorption of a star by a black hole is called a tidal disruption event (TDE) because of the powerful tidal forces that act on the star due to the black hole’s gravity.

As the star pulls closer and closer to the black hole’s mouth, the black hole’s tidal forces strip and stretch the star layer by layer; turning it into a long, noodle-like thread that wraps tightly around the black hole like spaghetti around a fork, forming a ball of hot plasma. This is known as spaghettification.

This plasma rapidly accelerates around the black hole and turns into a huge jet of energy and matter, which produces a characteristic bright flash that can be detected by optical, X-ray and radio wave telescopes.

But AT2018hyz is unusual: not only has it waited three years after swallowing a star to emit a flare, but the speed of material ejected from its mouth is staggering. Most TDE streams move at 10% the speed of light, but the ejected stellar matter of AT2018hyz moves at 50% the speed of light.

“We have been studying TDEs with radio telescopes for over a decade and occasionally find them glowing in radio waves as they erupt material when the star is first being swallowed up by the black hole. But there was radio silence in AT2018hyz for the first three years, and now it has brightened up dramatically and has become one of the brightest TDEs ever observed,” said Edo Berger, co-author of the study, professor of astronomy at Harvard University.

Scientists aren’t sure what causes the flash delay, but they think the delay may be more common than previously thought. To test if this is the case, astronomers will need to look at the sources of other TDEs previously thought to be out of commission to see if they can catch their flare again.

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