Supermassive black holes are notoriously messy when devouring a star, but they can also linger over their meals, letting out massive radio “burps” months or even years after their cosmic feast appears finished.
Now, scientists tracking these events have found there is no one-size-fits-all model for how black holes digest stellar material. Speaking Monday (June 15) at the 248th meeting of the American Astronomical Society in California, Kate Alexander, an astronomer at the University of Arizona who has been studying these events, said the behavior depends instead on their shifting dietary phases.
“Sometimes, after it seems like they are done eating, they may get indigestion and they may let out a large radio ‘burp,'” Alexander said during a press conference on Monday. “These late-time radio burps can appear when the black hole eats too fast or eats too slowly, so you should always eat the right speed if you want to avoid indigestion.”
Her recent research focuses on Tidal Disruption Events, or TDEs, which are cosmic catastrophes that occur when an unlucky star wanders too close to a supermassive black hole. As the star nears the behemoth, intense gravitational fields shred it into a spaghetti-like stream of gas debris in a process known as “spaghettification.”
Because these events are rare, occurring roughly once every 100,000 years in any given galaxy, astronomers must monitor a large number of galaxies just to spot them. Historically, targeted radio follow-up of these disruptions ceased if no emission was detected within the first year or so, leaving their long-term behavior unstudied.
“When we first started looking at them, we just stopped looking,” she said. “But, it turns out that we should have kept looking, because this is often when some of the most really interesting things are happening.”
Over the past six years, astronomers have been using the Karl G. Jansky Very Large Array (VLA) telescope in New Mexico to conduct the first large-scale, systematic radio observations of several dozen nearby TDEs. A 2024 paper, by radio astronomer Yvette Cendes of the University of Oregon and co-authored by Alexander, first reported that roughly 40% of all TDEs are detected in radio months to years after the initial disruption, long after the visible light has dimmed.
Now, published this year in The Astrophysical Journal, the new study led by Alexander sets out to explain why these long-dormant systems reactivate. To solve the mystery, the researchers combed through decades of data, analyzing 91 TDE candidates discovered between 1990 and 2019 before narrowing their focus to a gold-standard sample of 31 events with comprehensive, multiwavelength tracking.
By blending VLA radio data with archival optical and ultraviolet observations, plus fresh follow-up X-ray measurements, the team mapped how much gas the black holes actually consumed at any given point in time. Matching that feeding timeline against the exact moments the radio flares emerged revealed precisely how fast the black holes were eating when they unleashed their outflows, Alexander explained during the press briefing.
The data revealed that these delayed flares ignite at two opposite extremes, either while the black hole is rapidly overgorging on gas, or after its feeding rate has slowed to a crawl. In both scenarios, a fraction of the incoming gas is flung outward instead of being fully consumed, the team found. This expelled material then slams into the gas surrounding the black hole, triggering particle-accelerating shock waves that produce the radio emissions — effectively creating the cosmic “burps.”
This cosmic feeding mechanic operates identically across all scales, working the exact same way whether the black hole is a relative lightweight or a behemoth millions of times more massive than our sun, Alexander noted.
“For those of us who are astrophysicists,” she said, “this is really cool because we are now starting to understand how physics operates in these very different mass regimes.”
The team also found that TDEs destined to flare up later leave a distinct chemical fingerprint in their early optical spectra in the form of helium emission lines. This signature indicates that the star’s shredded debris is taking its time settling into a tidy, ingestible disk around the black hole — virtually guaranteeing a delayed case of cosmic indigestion, said Alexander.
“These are the black holes that are having longer lasting meals,” she said.
Based on these findings, the team suggests that a window of two to six years post-discovery is the most productive timeframe to hunt for these late-rising radio signals.
Ultimately, the team says the predictive chemical blueprint could serve as an invaluable screening tool. By filtering out the quiet eaters early on, astronomers can maximize highly competitive telescope time, focusing precious resources on the black holes most likely to put on a late-stage show.