In October 2018, a tiny star was ripped apart when it got too close to a black hole in a galaxy 665 million light-years from Earth. While it may sound exciting, the event came as no surprise to astronomers who sometimes witness these violent incidents scanning the night sky.
But nearly three years after the massacre, the same black hole is once again lighting up the sky – and it hasn’t swallowed anything new, scientists say.
“It took us completely by surprise – no one had ever seen anything like this before,” says Yvette Cendes, research associate at the Center for Astrophysics | Harvard & Smithsonian (CfA) and lead author of a new study analyzing the phenomenon.
The team concludes that the black hole is now ejecting matter traveling at half the speed of light, but don’t know why the exit was delayed for several years. The results, described this week in the Astrophysical Journalmay help scientists better understand the feeding behavior of black holes, which Cendes likens to a “burp” after a meal.
The team spotted the unusual outburst while revisiting tidal disturbance events (TDEs) – when invading stars are spaghettified by black holes – that have occurred over the past few years.
Radio data from the Very Large Array (VLA) in New Mexico showed the black hole mysteriously reanimated in June 2021. Cendes and the team rushed to take a closer look at the event.
“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,” says Cendes. . “All 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, the Chandra X-Ray observatory and the Neil Gehrels. Swift Observatory in space.
The radio observations of the TDE proved to be the most striking.
“We’ve been studying TDEs with radio telescopes for over a decade, and sometimes we see them glowing in radio waves as they spit out material as the star is first consumed by the black hole” , explains Edo Berger, professor of astronomy at Harvard University and the CfA, and co-author of the new study. “But in AT2018hyz there was radio silence for the first three years, and now it has brightened up dramatically to become one of the most radio-bright TDEs ever seen. »
Sebastian Gomez, postdoctoral fellow at the Space Telescope Science Institute and co-author of the new paper, said AT2018hyz was “mundane” in 2018 when he first studied it using visible light telescopes, including the Fred Lawrence’s 1.2m telescope. Whipple Observatory in Arizona.
Gomez, who was working on his doctoral dissertation with Berger at the time, used theoretical models to calculate that the black hole-torn star was only one-tenth the mass of our Sun.
“We monitored AT2018hyz in visible light for several months until it disappeared, then we forgot about it,” Gomez says.
TDEs are well known to emit light when they occur. When a star approaches a black hole, gravitational forces begin to stretch or spaghettify the star. Eventually, the elongated material orbits the black hole and heats up, creating a flash that astronomers can spot millions of light-years away.
Some spaghetti materials are sometimes sent back into space. Astronomers liken this to black holes which are messy eaters – not everything they try to consume makes it into their mouths.
But the outflow, known as the outflow, normally develops quickly after a TDE appears, not years later. “It’s as if this black hole suddenly started ejecting a bunch of material from the star that it ate up years ago,” Cendes says.
In this case, the burps are resounding.
The flow of matter moves as fast as 50% of the speed of light. By comparison, most TDEs have outgoing flux moving at 10% the speed of light, Cendes says.
“This is the first time we’ve seen such a long delay between power-in and power-out,” says Berger. “The next step is to determine if this is actually happening more regularly and we just haven’t looked 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 York University and Mark Wieringa of the Australia Telescope National Facility.