In June 2021, a radio telescope in New Mexico discovered that a black hole had begun releasing material into space, a phenomenon that in astrophysics is the equivalent of someone vomiting up food . The strange thing, however, is that it hadn't "eaten" anything in years.
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A team of astronomers observed the ejection event using six observatories: four in North America, South America, Africa and Australia , and two in space. The team linked the 2021 ejection to a star being consumed by the black hole three years ago. The team's research is published this week in The Astrophysical Journal.
"It completely surprised us, no one has ever seen anything like it before," said Yvette Cendes, a researcher at the Harvard & Smithsonian Center for Astrophysics and lead author of the study.
The phenomenon is called AT2018hyz. It is a tidal disruption, which occurs when a star that passes too close to a black hole is torn off, or undergoes what is known as spaghettification. The stellar material then orbits the black hole in its accretion disk, rather than being sucked in immediately. This material heats up, creating signals that astronomers on Earth.
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Sometimes tidal disruption events are rapid. Other times, stars are torn apart slowly as they orbit black holes.
Black holes eat indiscriminately, so it's not unusual for them to spew stellar material back into space. But three years is an unusually long time for a black hole to hold onto its "meal." Normally, when a star gets spaghetti-edged by a black hole, the results are pretty immediate.
In this case, the black hole held onto the stellar material for years before ejecting it at half the speed of light, extremely fast, as the outflow from most tidal disruption events travels at about 10% of the speed of light. "It's as if this black hole has suddenly started to eject a bunch of material from the star it ate years ago," Cendes said.
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The team believes that delayed outflows like AT2018hyz may be more common than previously thought. To figure this out, astronomers will need to examine the locations of more tidal disruption events that have already escaped their notice. It could mean that these supposed events have even more action in their future.
