A distant black hole shredded a star in 2018 and two years later released a plume of plasma traveling at around 50% the speed of light - astronomers aren't sure why it took so long.
|In this artist's illustration, a supermassive black hole spaghettifies and devours a star. Science Communication Lab, DESY|
A black hole devoured a star and two years later let out a massive belch. Astronomers were surprised by the delay between the cosmic meal and the blast of plasma that followed, and they aren't sure why it took so long.
In 2018, astronomers discovered evidence of a tidal disruption event caused by a black hole more than 650 million light years away ripping apart a star. Then, in 2020, 2021, and 2022, another team of researchers led by Yvette Cendes at the Harvard-Smithsonian Center for Astrophysics in Massachusetts used several radio telescopes to take another look.
During tidal disruption events, the black hole's powerful gravity rips up a star that gets too close, and the star's remains are dragged into a halo of debris known as an accretion disc before falling into the black hole. Occasionally, shortly after the star is shredded, that disc blasts out a spray of material.
"The action usually takes place in the first few months," Cendes says. "Normally, when we observe a tidal disruption [in radio wavelengths], we see an outflow about 20% of the time in the first few months, and if you don't see anything, radio telescope time is precious, so you move on and look at new things."
Observing later paid off in this case. Approximately two years after the tidal disruption event, an extremely bright plume of material began to blast away from the black hole at speeds up to half the speed of light. According to Cendes, the black hole almost certainly did not consume any other stars or eject any other material in the intervening time. If it had, observations from telescopes capable of viewing large areas of the sky at once would have detected it.
We have no idea why this belch was so late, she says. There are a few possible explanations, most of which have to do with the accretion disc's properties, but none of them quite fit. Cendes believes that determining what happened is especially important because these delayed outbursts could be occurring all over the universe.
"This was one of about two dozen events we observed, and so far, it appears that delayed outflows like this may be more common than we expected," Cendes says.
The Astrophysical Journal, DOI: 10.3847/1538-4357/ac88d0