Astronomers have spotted a black hole mysteriously spitting out pieces of a devoured star several years after consuming it.
The event, which scientists have classified as AT2018hyz, began in 2018 when astronomers saw the black hole to trap an unfortunate star in its strong gravitational pull before shredding it into pieces. Then, three years later, in 2021, a radio telescope in New Mexico picked up a signal indicating unusual activity – the black hole had begun ejecting the star at half the speed of light.
Black holes have been spotted before gobbling up stars before spewing them out, but so far the ejection has only ever happened at the same time as the meal. Researchers used four ground observatories located around the globe and two observatories in space to spot the event; they published their findings on October 11 in The Astrophysical Journal (opens in a new tab).
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“It took us completely by surprise – no one has ever seen anything like this before,” the lead author said. Yvette Cendes (opens in a new tab)an astrophysicist from the Harvard and Smithsonian Center for Astrophysics, said in a statement.
Black holes are messy eaters who like to play with their food. The consumption of a star by a black hole is called a tidal disturbance event (TDE) because of the powerful tidal forces acting on the star from the black hole’s gravity. As the star is coiled 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 string that wraps tightly around the black hole like spaghetti around a fork to form a ball of hot plasma. This is called spaghettification. This plasma rapidly accelerates around the black hole and turns into a huge jet of energy and matter, which produces a distinctive flash of light that optics, X-rays and radio wave telescopes can detect.
But AT2018hyz is unusual: not only did it wait three years after nibbling the star to emit a flash, but the speed of matter sent flying from its mouth is staggering. Most outflows from TDE travel at 10% the speed of light, but stellar matter ejected from AT2018hyz travels as fast as 50% the speed of light.
“We’ve been studying TDEs with radio telescopes for over a decade, and sometimes we see them glowing in radio waves as they spit out material as the star is first consumed by the black hole.” , co-author of the study Edo Shepherd (opens in a new tab), professor of astronomy at Harvard University, said in the statement. “But in AT2018hyz there was radio silence for the first three years, and now it’s brightened dramatically to become one of the most radiobright TDEs ever seen.”
Cendes thinks he might be late kicking out his previous meal. “It’s as if this black hole suddenly started ejecting a bunch of material from the star it ate years ago,” Cendes added.
Researchers don’t know exactly what causes flash delay, but they think delay may be more common than previously thought. To test if this is the case, astronomers will need to examine the sources of other TDEs, previously thought to be out of order, to see if they can catch them flashing once again.
“This is the first time we’ve seen such a long delay between feed and output,” Berger said. “The next step is to determine if this is actually happening more regularly and we just haven’t looked at TDEs late enough in their evolution.”