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September 7th, 2023 
Awake Goy 
Scientists have spotted black holes belching out the remains of stars that they ate years earlier, a discovery that upends our understanding of how these cosmic behemoths interact with the matter around them, reports a new preprint study.
After witnessing black holes unexpectedly glowing in radio light, astronomers gathered long-term observations of the exotic objects. The results revealed that the weird radio patterns are fueled by bits of star-stuff that are regurgitated years after they were first devoured by black holes, for reasons that remain mysterious.
Nothing can escape from inside the border of a black hole, known as an event horizon, but things are more flexible just outside this point-of-no-return. For instance, when a black hole gobbles up stars, or other matter, during what’s known as a tidal disruption event (TDE), much of its meal is lost to space. The remaining material gets stretched into noodly strings as part of a process called “spaghettification,” and eventually settles into a superheated disk that surrounds the black hole.
TDEs are known for producing radiant bursts of optical light that are fueled by the intense gravitational interactions between black holes and the matter they capture and consume. Models predict that these fireworks cease a few weeks after the onset of a TDE, once the spaghettified stars are destroyed and incorporated into the revolving disk around a black hole.
Now, scientists led by Yvette Cendes, a radio astronomer at the Center for Astrophysics | Harvard & Smithsonian, have discovered that black holes frequently “burp” up stellar material hundreds or thousands of days after the consumption of a star. The team concluded that “late radio emission marks a fairly ubiquitous but heretofore overlooked phase of TDE evolution,” according to a study posted on the preprint server arxiv that has not yet been peer-reviewed, though it has been submitted for publication in the Astrophysical Journal.
“It’s been very exciting, that’s for sure!” Cendes said, referring to the results, in an email to Motherboard. “It was originally a fishing expedition of sorts to see what was out there, because there have been one or two previously published TDEs that turned ‘on’ in radio hundreds of days post-event. But it’s one thing to find an unusual event and another to discover a phenomenon like this is common and just overlooked!”
“Reducing the initial data, I discovered three that had turned ‘on’ in just 24 hours,” she added. “That was pretty wild.”
Scientists have speculated about the dynamics of TDEs for decades, but these cataclysms are now coming into sharper focus thanks to sophisticated telescopes. In their new study, Cendes and her colleagues gathered long-term observations of TDEs with the Karl G. Jansky Very Large Array (VLA) in New Mexico, the MeerKAT radio telescope in South Africa, and the Australian Telescope Compact Array (ATCA) in New South Wales.
“Our sample of TDEs with radio observations is the largest to date: we observed 24 optically-selected TDEs,” said the team in the study, noting that one of those TDEs was the subject of a separate paper published in 2022. “In this paper we identify 9 new TDEs that had constraining radio upper limits at early times, and then exhibit brighter radio emission hundreds of days post optical discovery.”
“Of the 11 TDEs with transient radio emission, 9 TDEs were detected for the first time despite a lack of radio emission at earlier times, and 2 TDEs…were detected to significantly brighten at late time relative to their declining radio emission at earlier times,” the team added. “We find ≈ 40% of TDEs in this work show late-time emission, meaning a substantial fraction of all optically selected TDEs exhibit radio emission that rises on timescales of hundreds of days.”
In other words, almost half of the black holes in this dataset lit up with bright radio emission between two and six years after they ate a star, opening a new window into the intense environments surrounding black holes. The observations contradict most models of TDEs, which generally predict that stellar spaghetti provides a steady flow of “food” from the disk to the black hole, with little opportunity for hiccups and belches.
It’s wild to imagine letting out a little burp and realizing it originated from a random sandwich you ate in 2017. But that’s essentially what is going on with these black holes, albeit on a much larger cosmic level. Now that Cendes and her colleagues have detected this delayed brightening effect, they hope to figure out the unknown mechanisms that allow this material to escape black holes after such an extended delay. A forthcoming generation of sensitive telescopes, such as the Vera C. Rubin Observatory and the Square Kilometer Array, could help to unravel this mystery.
“This is exciting because it opens a new door to black hole physics,” Cendes said. “We can’t build an experiment on Earth that’s like the environment around a black hole, so we have to rely on observations like these. Frankly, we don't know yet where it’ll take us.”
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