Astronomers have detected a black hole mysteriously spewing 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 trap an unlucky star in his fort gravitational pull before cutting 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 belching out the star at half the speed of light.
Black holes have previously been seen gobbling up stars before regurgitating them, but so far, the ejection has only happened at the same time as the meal. The researchers used four Earth-based observatories located around the world and two space-based observatories to detect the event; published their findings on October 11 a Astrophysics Magazine (opens in a new tab).
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“This took us completely by surprise – no one had ever seen anything like this before,” said lead author Yvette Cendes (opens in a new tab), an astrophysicist at the Harvard-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 disruption event (TDE) because of the powerful tidal forces acting on the star due to the black hole’s gravity. As the star winds closer and closer to the black hole’s maw, the black hole’s tidal forces strip and stretch the star layer by layer; transforming it into a long noodle-like string that coils tightly around the black hole like spaghetti around a fork to form a ball of hot plasma. This is known as spaghettification. This plasma rapidly accelerates around the black hole and spins into a huge jet of energy and matter, producing a distinctive bright flash that optical, X-ray, and radio-wave telescopes can detect.
But AT2018hyz is unusual: not only did it wait three years after snacking on the star to emit a flash, but the speed of the material sent flying from its mouth is astonishing. Most TDE outflows travel at 10% the speed of light, but the stellar matter ejected from AT2018hyz travels at up to 50% the speed of light.
“We’ve been studying TDEs with radio telescopes for more than a decade, and sometimes we find them glowing in radio waves as they emit material as the star is first consumed by the black hole,” study co-author Edo Berger ( opens new tab), a professor of astronomy at Harvard University, said in the statement. “But AT2018hyz was radio silent for the first three years, and now it has brightened dramatically to become one of the brightest radio TDEs ever observed.”
Cendes thinks he might be belatedly expelling his previous meal. “It’s as if this black hole has suddenly started belching out a bunch of material from the star it ate years ago,” Cendes added.
Researchers aren’t sure what’s causing the flash delay, but they believe this postponement could be more common than previously thought. To test whether this is the case, astronomers will need to look at the sources of other TDEs, previously thought to be out of action, to see if they can make them flicker again.
“This is the first time we’ve seen such a long delay between feed and output,” Berger said. “The next step is to explore whether this actually happens more regularly and we simply haven’t been looking at TDEs late enough in their evolution.”