Evrim Yazgin
Cosmos science journalist
A supermassive black hole has done something that has dumbfounded astronomers.
In 2018 astronomers witnessed the black hole “spaghettifying” a star which came too close to the cosmic giant. This violent incident is pretty standard fair for black holes and has been seen a number of times.
The same black hole, located 665 million light years from Earth, has lit up again – but this time it hasn’t swallowed anything. It spat it back out!
“This caught us completely by surprise — no one has ever seen anything like this before,” says Yvette Cendes, a research associate at the Harvard and Smithsonian Centre for Astrophysics. Cendes is the lead author of a new study analysing the phenomenon published in the Astrophysical Journal.
While the team believes the black hole is now ejecting material at around half the speed of light, they’re unsure of why it took several years for this to happen. Their research may help scientists better understand how black holes feed. This latest phenomenon has been likened by Cendes to “burping” after a meal.
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The event was caught when the team revisited tidal disruption events (TDEs) that occurred over the last several years. TDEs occur when a star wanders close enough to a supermassive black hole to be torn apart by tidal forces.
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Radio data from the Very Large Array (VLA) in New Mexico showed the black hole mysteriously reanimated in June 2021.
“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 said. “All the applications were immediately accepted.”
They collected observations of the TDE, known as AT2018hyz, in multiple wavelengths, but the radio observations proved the most striking.
“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,” says co-author Edo Berger, professor of astronomy at Harvard University and the CfA. “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.”
Before revisiting the TDE, the supermassive black hole’s activity was relatively “unremarkable.”
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The tidal forces during a TDE lead to an accretion flow where the gravitational pull of the black hole “spaghettifies” the star. The elongated material spirals around the black hole and heats up, creating a flash which travels across the universe.
Sometimes, some of the material spinning around the black hole is flung into space. Astronomers – who appear to love anthropomorphising black holes – liken this to ‘messy eating.’ But this outflow normally occurs very quickly after a TDE, not years later, like this latest emission.
And the supermassive black hole’s “burp” was a monster. Normally, TDE outflows travel at around 10% the speed of light. This emission is travelling as fast as 50% lightspeed.
“This is the first time that we have witnessed such a long delay between the feeding and the outflow,” Berger says. “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.”