In October 2018, a small star was ripped to shreds when it wandered too close to a black hole in a galaxy located 665 million light years away from Earth.
Though it may sound thrilling, 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 event the same black hole is lighting up the sky, and scientists believe that they’re witnessing the outflow of that star’s remnants years after it would normally happen, prompting a whirlwind of questions.
“This caught us completely by surprise—no one has ever seen anything like this before,” says Yvette Cendes, a research associate at the Center for Astrophysics at Harvard and lead author of a new study analyzing the phenomenon.
The team concludes that the black hole is now ejecting material traveling at half of the speed of light, but are unsure why the outflow was delayed by several years. The results, described this week in the Astrophysical Journal, may help scientists better understand black holes’ feeding behavior, which Cendes likens to “burping” after a meal.
The team spotted the unusual outburst while revisiting tidal disruption events (TDEs)—when encroaching stars are pulverized by black holes—that occurred over the last several 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.
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, and the Chandra X-Ray Observatory and the Neil Gehrels Swift Observatory in space.
Radio observations of the TDE 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 Edo Berger, also at Harvard University and co-author on the new study.
“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.”
TDEs are well-known for emitting light when they occur. As a star nears a black hole, gravitational forces begin to stretch, or spaghettify, the star. Eventually, the elongated material spirals around the black hole and heats up, creating a flash that astronomers can spot from millions of light years away.
Some spaghettified material occasionally gets flung out back into space. But the emission, known as an outflow, normally develops quickly after a TDE occurs, not years later.
Taking the conversation to Reddit, the astronomers were asked if the phenomenon was like how in a food processor, there’s a ring of material always just outside the reach of the blades.
“Basically yea,” Cendes replied. “This material was in an accretion disc surrounding the black hole after [the star] was unbound. In 20% of cases you then see a radio outflow at the part where it’s torn apart.”
The outflow of material is traveling as fast as 50% the speed of light. For comparison, most TDEs have an outflow that travels at 10% the speed of light, Cendes says.
“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.”