The Black Holes That Burp Years After They Eat
Featured image: [Artist’s concept of a tidal disruption event showing a star being spaghettified and a delayed outflow; credit: NASA/CXC/M.Weiss]
When a star strays too close to a supermassive black hole, the result is violent: tidal forces shred the star into a stream of gas in a process called spaghettification, producing a brilliant flare that can outshine the black hole’s entire host galaxy. For decades, astronomers assumed the show ended when that initial flare faded.
They were wrong.
A team using the Karl G. Jansky Very Large Array in New Mexico has conducted the first systematic long-term radio survey of tidal disruption events, monitoring 31 such events over years. The results, presented at the American Astronomical Society’s 248th meeting in June 2026 and published in The Astrophysical Journal, show that roughly 40 percent of these black holes produce a delayed radio outburst months or even years after the initial optical flare.
“Sometimes, after it seems like they are done eating, they may get indigestion and they may let out a large radio burp,” said Kate Alexander of the University of Arizona, lead author of the study.
A messy meal
When a supermassive black hole tears apart a star, roughly half of the stellar debris falls into orbit around the black hole, forming a hot accretion disk that powers the initial optical, ultraviolet, and X-ray flare. Astronomers had assumed the rest of the story was straightforward: the disk slowly drains into the black hole, the light fades, and the galaxy returns to darkness.
The VLA survey tells a different story. The black hole does not swallow its meal cleanly. Some of the infalling gas is flung back out in jets or winds launched from close to the event horizon. When that expelled material slams into the gas surrounding the black hole, it sets off shock waves that accelerate particles and generate synchrotron radio emission. The burp, in effect, is the sound of a messy eater spitting part of its dinner back into the room.
Yvette Cendes of the Center for Astrophysics at Harvard and Smithsonian, lead author of a companion paper that first identified the prevalence of delayed radio emission, noted the timescales involved. Her team’s 2024 study found that outflows are launched 500 to 2,000 days after the initial disruption, with velocities between 2 and 15 percent of the speed of light and kinetic energies of 10 to the 47th to 10 to the 49th erg.
Two kinds of burps
The survey revealed two distinct patterns of delayed emission. Some events produce a radio flare within a few hundred days, while the black hole is still rapidly accreting stellar debris. Others flare much later, years after the event, when the feeding has slowed to a trickle.
“It turns out that wildly different feeding rates can drive the same bright radio outburst,” Alexander said. “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.”
The finding rewrites the observational strategy for studying tidal disruption events. Earlier surveys typically stopped monitoring a year after discovery, concluding that events without early radio emission were radio-quiet. The VLA data prove that the most interesting radio behavior often begins years later, not earlier.
The helium fingerprint
The team also identified a predictive signature. Tidal disruption events that later produce delayed radio flares are less likely to show helium emission lines in their early optical spectra. These helium-poor events indicate that the shredded star’s debris is taking its time settling into a disk around the black hole.
“These are the black holes that are having longer lasting meals,” Alexander said. The finding gives astronomers a practical shortlist of which newly discovered tidal disruption events are worth monitoring for years. Events that are helium-poor at early times are the best candidates for long-term radio follow-up.
The optimal observation window, the study concludes, is two to six years after the initial optical discovery. Events monitored within this window are the most likely to catch the delayed radio flare, when the black hole’s messy meal finally makes itself heard across the radio spectrum.
Scale-invariant physics
The discovery has implications beyond tidal disruption events themselves. The same feeding and outflow dynamics appear to operate across all black hole mass scales, from stellar-mass black holes to the supermassive giants at the centers of galaxies. Tidal disruption events offer a rare opportunity to watch a supermassive black hole’s feeding rate change in real time, providing a laboratory for testing theories of accretion and feedback that are otherwise impossible to observe on human timescales.
For Alexander and her team, the message to the astronomical community is clear: keep watching. “We used to think the show was over once the optical light faded,” she said. “Fortunately we kept looking, and now the NSF VLA is showing us that the black hole can come back years later with a dramatic encore performance in radio light.”