A supermassive black hole does not always go quiet after it tears apart a star. New observations show that some of these cosmic giants can keep emitting radio waves for months or even years after the destruction event, a delayed signal researchers have described as a kind of cosmic “burp.”
The finding changes how astronomers understand tidal disruption events, or TDEs, which happen when a star wanders too close to the center of a galaxy and is pulled apart by the black hole’s extreme gravity. What was once treated as a short-lived burst of activity may, in many cases, be followed by a much later phase of renewed emission.
When a Star Gets Too Close
In a TDE, the star is stretched and shredded in a process known as spaghettification before its material forms a disk around the black hole. Some of that gas falls inward and is swallowed, but not all of it disappears immediately.
These events are rare, with astronomers estimating that a TDE happens only about once every 100,000 years in a typical galaxy. That rarity is one reason researchers must monitor many galaxies to catch enough cases for study.
Why the Delayed Radio Signal Matters
The “burp” analogy refers to radio emission that appears well after the initial star-destruction flare. According to Kate Alexander of the University of Arizona, the new research, published in The Astrophysical Journal, shows that delayed radio waves can emerge when gas that was not fully swallowed gets thrown back out.
That expelled material collides with surrounding matter and creates shock waves. Those shocks accelerate energetic particles, producing radio signals that telescopes can detect long after the visible light has faded.
Long-Term Monitoring Reveals a Bigger Pattern
For years, astronomers often stopped looking for radio activity if nothing appeared within the first year. That approach likely missed a large share of the late-stage behavior now coming into view.
Using the Karl G. Jansky Very Large Array in New Mexico, researchers followed dozens of TDEs in nearby galaxies over the past six years. They found that about 40 percent of the events produced radio waves several months to several years after the star was destroyed.
What the Data Showed
To trace the source of the delayed signals, the team examined 91 TDE candidates identified between 1990 and 2019 and then selected 31 events with the most complete data. They combined radio observations from the VLA with optical, ultraviolet, and X-ray measurements to build a fuller picture of each event.
The analysis pointed to two different situations that can lead to late radio emission. In one case, the black hole consumes large amounts of gas very quickly, while in the other the intake rate drops sharply.
| Finding | Detail |
|---|---|
| Delayed radio signals | Seen months to years after the star-disruption flare |
| Sample size | 91 candidates reviewed, 31 examined in depth |
| Observation result | About 40 percent of monitored TDEs showed late radio emission |
In both cases, some stellar material still escapes being swallowed. That leftover gas then slams into the surrounding environment, generating the shock and radio emission that reveal the black hole is still active.
A Longer View of Black Hole Feeding
The study suggests that supermassive black holes do more than pull in matter at the moment of destruction. They can leave behind a trail of activity that continues well after the brightest phase has ended.
For astronomers, that means TDEs may need to be observed for much longer than before to capture the full sequence of events. The late radio “burp” is now emerging as an important clue to how black holes interact with gas, energy, and their galactic surroundings.
As the data show, the most revealing part of a black hole’s meal may arrive only after the main course has already vanished from view.