Astronomers studying tidal disruption events have solved the mystery of why supermassive black holes launch delayed radio “burps” months or years after devouring a star. Research reveals these powerful outflows ignite during two opposite dietary extremes: when black holes overgorge on gas, or when their feeding rate slows to a crawl.
Dr. Adelle Goodwin and Kate Alexander on Tidal Disruption Events
Supermassive black holes are notoriously messy when consuming stars, but their long-term behavior has puzzled astrophysicists for years. When a star wanders too close to a supermassive black hole, intense gravitational fields shred it into a spaghetti-like stream of gas debris in a process known as spaghettification. According to Dr. Adelle Goodwin, an astrophysicist at Curtin University in Western Australia and a Forrest Research Foundation fellow, You have to get really, really close to a black hole to get to the event horizon
where gravity is too strong for light and matter to escape, and Stars get destroyed further out than that.
Goodwin also noted that only about half of the star that gets too close to the back hole will end up eventually being swallowed by that black hole,
while the other material is launched back into space in powerful jets and outflows, sometimes so large they can influence the entire evolution of a galaxy.
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. Kate Alexander, an astronomer at the University of Arizona, noted that astronomers simply stopped looking when nothing appeared early on. However, recent systematic observations have upended that assumption, proving that some of the most interesting activity happens long after visible light has dimmed.
Karl G. Jansky Very Large Array Observations in The Astrophysical Journal
Over the past six years, astronomers have used 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 tidal disruption events, or TDEs. In a study published in The Astrophysical Journal, a team led by Alexander examined 91 TDE candidates discovered between 1990 and 2019, ultimately focusing on a gold-standard sample of 31 events with comprehensive multiwavelength tracking.
By blending VLA radio data with archival optical and ultraviolet observations, alongside fresh follow-up X-ray measurements, the researchers mapped how much gas the black holes consumed at any given time. In separate research published in Nature Astronomy, Goodwin and co-author Dr. Andrew Mummery from the Institute for Advanced Study utilized radio telescopes to study 20 tidal disruption events, noting that radio is the only frequency where we can watch the jets and outflows as they’re moving outwards
according to Goodwin.
American Astronomical Society Meeting Findings and Nature Astronomy
The combined data revealed that these delayed flares ignite at two opposite extremes. According to Alexander’s findings presented at the 248th meeting of the American Astronomical Society in California, late-time radio burps appear when a black hole eats either too fast or too slowly. Goodwin and Mummery’s research in Nature Astronomy further quantified this, showing that supermassive black holes—which range in size from hundreds of thousands to billions of times the mass of our sun—fire off powerful jets in two distinct phases: first, when feeding at very high rates, and second, hundreds to thousands of days after the star is torn apart when the feeding rate drops to about 2% of the maximum rate at which a black hole can swallow material.
This 2% threshold is already recognized for triggering burps from much smaller stellar-mass black holes, which are much smaller bodies about 10 to 50 times the mass of our sun. As Alexander explained, a fraction of the incoming gas is flung outward rather than being fully consumed in both scenarios. This expelled material slams into the gas surrounding the black hole, triggering particle-accelerating shock waves that produce radio emissions. Furthermore, the team found that TDEs destined to flare up later leave a distinct chemical fingerprint in their early optical spectra via helium emission lines, indicating that shredded stellar debris takes time to settle into a tidy disk.
Dr. Sara Webb on Supermassive Black Holes
The discovery that black holes release jets at the same point in their feeding cycle regardless of their size allows scientists to predict when such outflows will occur, which Goodwin said would narrow observation windows and free up precious telescope time. Dr. Sara Webb, an astrophysicist at Swinburne University who was not involved in the research, noted that the study demonstrates that supermassive black holes behave rather predictably at two distant periods in their evolution
and ties these findings back to what we’ve seen previously on the much smaller scale stellar-mass black holes
according to Webb, who also added that We are still trying to untangle the very fundamentals of the most extreme objects in the universe – supermassive black holes.
By utilizing helium emission lines as a predictive chemical blueprint, astronomers can screen out quiet eaters early on and narrow observation windows. 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 late-rising radio signals. This screening tool maximizes highly competitive telescope time, allowing researchers to focus precious resources on the systems most likely to produce late-stage shows.
