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In October 2018, Sebastian Gomez, then a Harvard Ph.D. student, observed a black hole tear apart and engulf a small star in a galaxy located 665 million light years away from Earth. Almost three years later, he and a team of researchers noticed that the same black hole had begun ejecting stellar material.

Though scientists had previously observed black holes expelling cosmic debris immediately after tearing stars apart, researchers affiliated with the Harvard-Smithsonian Center for Astrophysics noticed that the black hole — which had absorbed a star in 2018 — began ejecting stellar remnants in June 2021, much later than initially thought possible.

The team’s study, published two weeks ago in The Astrophysical Journal, explored the new phenomenon of delayed outflow of material emanating from a black hole as the result of what is known as a tidal disruption event, where a star strays too close to a black hole, whose tidal forces rip it into pieces.

The TDE described in the study, which the researchers have named AT2018hyz, was of interest to astronomers due to its peculiarly delayed “burping” behavior, according to Harvard Professor of Astronomy and study co-author Edo Berger.

“While this material is slowly draining, it is also possible that some of it will be blown off, and this is what we have seen in tidal disruption events so far,” Berger wrote in an email.

“However, in AT2018hyz, the material is being blown off (or burped) almost three years later, and that's the mystery,” he added. “We usually detect radio waves (which are a sign of material being blown off) right when a tidal disruption event occurs.”

Yvette N. Cendes, a research associate at the CfA and the study’s lead author, said she was also surprised at the long period before the emission.

“Nobody expected that that could happen in theory or really considered it,” Cendes said. “It's an awkward length of time.”

The unusual delay opens up new avenues of exploration in both experimental and theoretical research, revealing “a new dimension of our understanding of physics,” according to Cendes, who said the study’s recent developments have been “pretty exciting.”

“It’s the first time you realize, ‘Oh, this is something crazy that hasn't been seen before,’” she said. “It’s a pretty exciting moment, you know — the fact that you know this and nobody else really knows this on the planet.”

Berger added that the incident will likely compel astronomers to revisit some “basic assumptions” about TDEs, which he described as “the fun part” of astrophysics.

“We tried something new and unorthodox (looking at TDEs with radio waves years after the disruption) and made a new and unexpected discovery about the behavior of black holes that will now require detailed new models and computer simulations,” he wrote.

Cendes said researchers hope to expand on their investigation into the frequency of delayed burping behavior across other TDEs. She added that her team is still actively tracking the black hole's behavior as the light from the TDE continues to intensify.

“I can tell you that it's still getting brighter,” she said of the emission from the black hole. “Our last observation was last month, so it's going to be in our lives for a bit, and I'll definitely let everybody know what AT2018hyz is up to when the time is right.”