‘Never seen before’: Black hole violently rips apart star and ‘burps’ it out years later

In 2018, scients observed a black hole about 650 million light-years away ripping a star into shreds because it got too close. That was about par for the course as far as black holes go. But three years later, the same black hole lit up and spewed out matter, and it didn’t swallowing anything in between.
“This caught us completely surprise — no one has ever seen anything like this before,” said Yvette Cendes, a research associate at the Harvard and Smithsonian Centre for Astrophysics in a press statement, Cendes is a lead author of a research article published in The Astrophysical Journal.

The researchers report that the black hole is spitting out material that is travelling at half the speed of light but they are unsure why this outflow was delayed several years. It contradicts what scients know so far about black holes’ feeding behaviour, phenomena which Cendes likens to burping after a meal.
Accidental discovery
This unusual phenomenon was discovered when the research team was revisiting tidal disruption events (TDEs) that occurred over the last few years. A TDE refers to an event when a black hole violently rips apart stars that gets too close.
While going through radio data from the Very Large Array (VLA) in New Mexico, they found that the black hole mysteriously came back to life in June 2021. They then applied for “Director’s Discretionary Time” with various telescopes, which is a priority request for telescope time for events that can’t wait. According to Cendes, the applications were immediately accepted.
Detective work
After being granted time, the team collected observations of this particular TDE, named AT2018hyz in many different wavelengths using the VLA, the ALMA Observatory in Chile, MeerKAT observatory in South Africa, the Australian Telescope Compact Array and the Chandra X-Ray Observatory and the Neil Gehrels Swift Observatory in space.

“We have been studying TDEs with radio telescopes for more than a decade, and we sometimes find they shine in radio waves as they spew out material while the star is first being consumed the black hole. But in AT2018hyz there was radio silence for the first three years, and now it’s dramatically lit up to become one of the most radio-luminous TDEs ever observed,” said Edo Berger, co-author of the new study. Berger is a professor of astronomy at Harvard University and the Centre for Astrophysics.
The anomalies
It is known that TDEs emit light when they occur. When a star comes close to a black hole, the gravitational forces will stretch or “spaghettify” the star, according to the Centre for Astrophysics. Then, the elongated material spirals around the black hole, gets heated up, and creates a flash that can be detected us millions of light years away.
But black holes are messy eaters. Some of this spaghettified materially gets flung back into space sometimes. But this usually happens almost immediately after the TDE, not years later. “It’s as if this black hole has started abruptly burping out a bunch of material from the star it ate years ago,” explained Cendes.
Also, the outflow of material from TDEs usually travels at around 10 per cent the speed of light while in this example, it was going as fast as 50 per cent the speed of light, adding more questions to the mystery. The next step for the researchers is to explore such events further to understand whether this actually happens more regularly and whether astronomers simply have not been looking at TDEs late enough in their life cycle.