Astronomers can’t explain a black hole “burp” years after it ate a star


Black holes have been seen to chow down on stars that wander too close, resulting in a bright stellar show. But now a black hole has been seen doing something nobody’s ever seen before – it “burped up” material several years after eating a star, leaving astronomers baffled.

Black holes are famously ravenous objects, swallowing up everything that gets too close, including light itself. When stars are on the menu, the intense gravitational forces stretch the material out into long strands in an event known as “spaghettification” or more officially, a tidal disruption event (TDE). This produces clear signals of light, radio and other waves that astronomers can detect as bursts that last a few weeks or months.

In October 2018, astronomers detected and studied a TDE known as AT2018hyz. It seemed to be pretty average for this kind of event – a star just one-tenth the mass of the Sun was being swallowed by a black hole about 665 million light-years away. The light show faded over a few months, and astronomers didn’t really give it much thought after that.

That is until June 2021, when the black hole suddenly fired back up with radio signals. This emission appears to be an outflow, the result of material thrown back into space as the black hole shredded the star – but this usually shows up within days or weeks of the initial event, not years later.

“This caught us completely by surprise – no one has ever seen anything like this before,” said Yvette Cendes, lead author of the study. “It’s as if this black hole has started abruptly burping out a bunch of material from the star it ate years ago.”

These burps have a lot of power behind them, too. The team calculated that they’re traveling at around half the speed of light, which is about five times faster than most TDE outflows.

So far the astronomers can’t explain why it took so long for these radio signals to show up. But studying the feeding habits of black holes has revealed some unexpected anomalies in the past – one was seen to be slowly snacking on a star over 10 years, while another would slurp a layer off each time the star swung past, producing flashes that repeat like clockwork. More detailed observations of these events could help unravel how regular they may be occurring.

“This is the first time that we have witnessed such a long delay between the feeding and the outflow,” said Edo Berger, co-author of the study. “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.”

The research was published in The Astrophysical Journal.

New Atlas, 11 October 2022