Black holes continue to “burp” the stars they ate years ago

In an interview with Live Science, Yvette Sindis, a research associate at the Harvard-Smithsonian Center for Astrophysics and lead author of the study, described black holes as “chaotic eaters,” where some of the stellar material is ejected away from the black hole. The rest is formed into the accretion disc — a flat, elliptical structure surrounding the black hole — which feeds matter into the black hole. Disengagement takes only a few hours, Sindis said.

Traditionally, astronomers would only watch outflows for a few months after a TDE and still not notice anything. However, when double-checking the 24 black holes observed by astronomers, 10 of them “burped” stellar matter within two to six years after the TDEs occurred, according to Live Science. The observations were detailed in an August 25 study uploaded to the preprint database arXiv. Cendes also analyzed her findings on Reddit.

“If you look years later, a very, very large fraction of these black holes that had no radio emission at these early times will suddenly ‘turn on’ in radio waves,” Sindis told Live Science. “I call it ‘burping’ because we’re having a kind of lag where this material doesn’t come out of the accumulating disk until much later than people were expecting.”

It’s not clear why black holes start working again, but Cindis and her team have ruled out that it could come from inside black holes. Defining the outer edge of black holes is the boundary known as the event horizon, a point from which nothing, not even light, can escape. “Black holes are environments with very intense gravity even before they cross the event horizon, and that’s what’s really driving this,” Sindis told Live Science. “We don’t fully understand whether the material observed in the radio waves is coming from the accretion disk or if it is stored somewhere near the black hole.”

Equally puzzling are the detections of radio emitted by black holes shortly after the initial event, as they fade out, then brighten again. “There was a second peak, where the black holes rebrightened, and that’s completely new and unexpected,” Sindis told Live Science. “People thought there would be one outflow, and then it just kind of ended. So this observation means that these black holes can ‘turn on’ and then ‘turn on’ again.”