In October 2018, a tiny star shattered when it got too close to a black hole in a galaxy 665 million light-years from Earth. Although it may sound exciting, 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 nothing new has been swallowed, scientists say.
“This took us completely by surprise: no one had ever seen anything like this before,” he said. Yvette Cendes, associate researcher at the Center for Astrophysics | Harvard & Smithsonian (CfA) and lead author of a new study looking at the phenomenon.
The team concludes that the black hole is now ejecting material traveling at half the speed of light, but they are not sure why the exit was delayed by several years. The results, described in Astrophysical Journal, they can help scientists better understand black holes’ feeding behavior, which Cendes likens to “belching” after a meal.
The team detected the unusual outburst while reviewing tidal disruption events (TDEs), when invading stars are spaghettied by black holes, that have occurred in recent 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 apply for Director’s Discretionary Time on multiple telescopes, which is when you find something so unexpected that you can’t wait for the normal cycle of telescope proposals to observe it,” Cendes explained in a statement. “All requests were accepted immediately.”
The team collected observations of the TDE, designated 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, the Chandra X-ray Observatory, and the Neil Gehrels Observatory. Swift in space.
The TDE radio observations turned out to be the most surprising.
“We’ve been studying TDEs with radio telescopes for more than a decade, and sometimes we find that they glow in radio waves as they shed material as the black hole first consumes the star,” said Edo Berger, professor of astronomy at Harvard. University and CfA, and co-author of the new study. “But at AT2018hyz there was radio silence for the first three years, and now it brightened dramatically to become one of the brightest radio TDEs ever observed.”
Sebastian Gomez, a postdoctoral fellow at the Space Telescope Science Institute and co-author of the new paper, says AT2018hyz was “normal” in 2018 when he first studied it with visible-light telescopes, including the 1.2m telescope on the Space Telescope. Fred Lawrence of the Whipple Observatory in Arizona.
Gómez, who at the time was working on his doctoral thesis with Berger, used theoretical models to calculate that the star torn apart by the black hole was only a tenth the mass of our Sun.
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“We monitored AT2018hyz in visible light for several months until it faded and then we erased it from our minds,” Gomez said.
TDEs are well known for emitting light when they occur. As a star approaches a black hole, gravitational forces begin to stretch or scrunch the star. Eventually, the elongated material spirals around the black hole and heats up, creating a flash that astronomers can detect from millions of light-years away.
Occasionally, some spaghetti material is thrown into space. Astronomers compare it to black holes being messy eaters: not everything they try to consume makes it to their mouths.
But the broadcast known as outflow, it usually develops quickly after a TDE occurs, not years later. “It’s as if this black hole has abruptly started belching out a bunch of material from the star that it ate years ago,” Cendes explained.
In this case, the burps are resounding.
The outflow of material travels at a speed of 50 percent of the speed of light. For comparison, most TDEs have an outflow that travels at 10 percent of the speed of light, Cendes said.
“This is the first time we have witnessed such a long delay between feeding and departure,” said Berger. “The next step is to explore if this really happens more often and we just haven’t been looking at TDEs late enough in their evolution.”
Other study co-authors include Kate Alexander and Aprajita Hajela of Northwestern University; Ryan Chornock, Raffaella Margutti, and Daniel Brethauer of the University of California, Berkley; Tanmoy Laskar of Radboud University; Brian Metzger of Columbia University; Michael Bietenholz of the University of York and Mark Wieringa of the Australian National Telescope Facility.
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