A black hole spewed out the remains of a star years after consuming it

Three years ago, in October 2018, astronomers observed a black hole consuming and ripping a star apart in a galaxy 665 million light years away from Earth.

Now, only recently, the same astronomers observed the same black hole lighting the sky, despite the fact it hadn't sucked another star into its vicinity, a press statement reveals.

In a never-before-seen observation, the astronomers found that the black hole was ejecting, or regurgitating, the stellar material it consumed three years ago. The new finding could help the scientific community to better understand the feeding behavior of black holes and the crucial role it plays throughout the cosmos.

"No one has ever seen anything like this before"

A new study detailing the astronomers' findings suggests the black hole is ejecting these stellar remnants at half the speed of light. The researchers don't understand why it took three years for this to occur, and they don't know what processes would have been acted on the stellar remains throughout that time period.

"This caught us completely by surprise — no one has ever seen anything like this before," explained Yvette Cendes, a research associate at the Center for Astrophysics | Harvard & Smithsonian (CfA) and lead author of the study. Cendes added that the behavior could be compared to someone "burping" after a meal.

Cendes and her colleagues spotted the never-before-seen phenomenon while re-observing black holes that had recently eaten stars in a phenomenon known as a tidal disruption event (TDE). In radio data captured in June 2021 by the Very Large Array (VLA) in New Mexico, they saw that the black hole in question had mysteriously started reanimating.

“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,” Cendes said. “All the applications were immediately accepted.”

Thanks to that quick response, the team was able to collect data on the TDE, dubbed AT2018hyz, in numerous wavelengths of light using several state-of-the-art observatories, including the Chandra X-Ray Observatory, Neil Gehrels Swift Observatory, and the ALMA Observatory in Chile.

A "burping" black hole

TDEs are known to emit vast amounts of light when they occur, due to the material from the star stretching out and heating up as it's consumed. However, the emissions typically occur almost straight away — not years later.

"It’s as if this black hole has started abruptly burping out a bunch of material from the star it ate years ago," Cendes said.

Next, the researchers aim to investigate whether this is a phenomenon that occurs often, but just hadn't been observed before, or if they really witnessed an incredibly rare cosmic event in action. Follow-up studies could help to shed new light on the mysterious inner workings of the many black holes at the center of galaxies throughout the universe.