A black hole has been observed spewing out material from a star it shredded years earlier
In October 2018, a small star was ripped to shreds when it wandered too close to a black hole in a galaxy located 665 million light years away from Earth.
Though it may sound thrilling, the event did not come as a surprise to astronomers who occasionally witness these violent incidents while scanning the night sky.
But nearly three years after the massacre, the same black hole is lighting up the skies again — and it hasn’t swallowed anything new, scientists say.
Yvette Cendes is a research associate at the Center for Astrophysics | Harvard & Smithsonian (CfA) and lead author of a new study analysing the phenomenon. She said: “This caught us completely by surprise — no one has ever seen anything like this before.”
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black hole feeding behaviour
The team concluded that the black hole is now ejecting material travelling at half the speed of light, but they are unsure why the outflow was delayed by several years. The results may help scientists better understand black holes’ feeding behaviour, which Cendes likens to ‘burping’ after a meal.
The team spotted the unusual outburst while revisiting tidal disruption events (TDEs) — when encroaching stars are spaghettified by black holes — that occurred over the last several years.
Radio data from the Very Large Array (VLA) in New Mexico showed that the black hole had mysteriously reanimated in June 2021. Cendes and the team rushed to examine the event more closely.
“We applied for Director’s Discretionary Time on multiple telescopes, which is when you find something so unexpected, you can’t wait for the normal cycle of telescope proposals to observe it,” Cendes explains. “All the applications were immediately accepted.”
The team collected observations of the TDE, dubbed AT2018hyz, in multiple wavelengths of light using the VLA, the ALMA Observatory in Chile, MeerKAT in South Africa, the Australian Telescope Compact Array in Australia, and the Chandra X-Ray Observatory and the Neil Gehrels Swift Observatory in space.
Radio observations of the TDE proved the most striking.
Edo Berger is professor of astronomy at Harvard University and the CfA, and co-author on the new study. He said: “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 by 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.”
unremarkable
Sebastian Gomez, a postdoctoral fellow at the Space Telescope Science Institute and co-author on the new paper, says that AT2018hyz was ‘unremarkable’ in 2018 when he first studied it using visible light telescopes, including the 1.2-m telescope at the Fred Lawrence Whipple Observatory in Arizona.
Gomez, who was working on his doctoral dissertation with Berger at the time, used theoretical models to calculate that the star torn apart by the black hole was only one tenth the mass of our sun.
He said: “We monitored AT2018hyz in visible light for several months until it faded away, and then set it out of our minds.”
TDEs are well-known for emitting light when they occur. As a star nears a black hole, gravitational forces begin to stretch, or ‘spaghettify’, the star. Eventually, the elongated material spirals around the black hole and heats up, creating a flash that astronomers can spot from millions of light years away.
Some spaghettified material occasionally gets flung out back into space. Astronomers liken it to black holes being messy eaters — not everything they try to consume makes it into their mouths.
But the emission, known as an outflow, normally develops quickly after a TDE occurs — 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,” Cendes explained. In this case, the burps are resounding.
The outflow of material is travelling as fast as 50% the speed of light. For comparison, most TDEs have an outflow that travels at 10% the speed of light.
Berger added: “This is the first time that we have witnessed such a long delay between the feeding and the outflow.
“The next step is to explore whether this actually happens more regularly and we have simply not been looking at TDEs late enough in their evolution.”
The research is published in Astrophysical Journal.
Image: Artist’s illustration of tidal disruption event AT2019dsg where a supermassive black hole spaghettifies and gobbles down a star. Some of the material is not consumed by the black hole and is flung back out into space. © DESY, Science Communication Lab.
