Astronomers have discovered a universal rule governing black hole jet formation, revealing that stellar-mass and supermassive black holes launch powerful radio outflows at the same critical 2% Eddington feeding threshold, regardless of their vastly different sizes.
Black holes are notorious for defying intuition. Often compared with cosmic vacuum cleaners that indiscriminately suck in everything nearby, their actual feeding habits are considerably more violent and untidy. When a black hole encounters a star, it does not swallow the material neatly. Much of the stellar debris is violently expelled into space through massive outflows that can significantly affect the evolution of entire galaxies.
Now, an international team of researchers has uncovered a surprising regularity in how these cosmic engines operate. According to findings published in Nature Astronomy under the title A universal critical accretion rate for black hole jet formation,
black holes ranging from small stellar objects to enormous giants millions of times the mass of the sun follow the same physical rules when launching powerful radio jets into space.
Using Tidal Disruption Events as a Cosmic Shortcut
Confirming that black holes obey universal physics has historically proved exceptionally difficult. Changes surrounding supermassive black holes at the centers of galaxies typically unfold across thousands or even millions of years, making real-time observation nearly impossible within a human lifetime.
To bypass this temporal barrier, the research team focused on tidal disruption events—cataclysmic occurrences in which a wandering star ventures too close to a supermassive black hole and is ripped apart by intense gravitational forces. The resulting feeding episodes condense processes that normally take millennia into a window of just a few years.
“We really wanted to figure out this massive puzzle. Why do some supermassive black holes blast out radio jets right after shredding a star, while others just sit there looking completely dormant, only to suddenly fire up their jets months or even years later?”
Andrew Mummery, Institute for Advanced Study
The study was led by Andrew Mummery, a Martin A. and Helen Chooljian Member in the School of Natural Sciences at the Institute for Advanced Study, and Adelle Goodwin, a Forrest Research Foundation Fellow at Curtin University’s International Centre of Radio Astronomy Research in Western Australia. The collaboration combined data gathered by telescopes across America, Australia, India, South Africa, and space.
Narrowing Twenty Disruptions Down to Ten Key Systems
To test whether mass influences jet production, the astronomers assembled twenty tidal disruption events with publicly available observations spanning optical, ultraviolet, X-ray, and radio wavelengths. Radio measurements allowed the team to estimate the moment an outflow was launched rather than relying solely on when its radiation became detectable.
After filtering out events lacking adequate late-time ultraviolet or optical measurements, the team narrowed their sample down to 10 tidal disruption systems containing 11 radio flares with robust data constraints. Analyzing optical, ultraviolet, and X-ray data with time-dependent relativistic accretion-disk calculations enabled the researchers to estimate how quickly the black holes consumed material during outflow launches.
Two Distinct Feeding Phases and the Two Percent Rule
Rather than clustering randomly around a single feeding rate, the analyzed radio outflows separated cleanly into two distinct groups corresponding to different stages of the black hole feeding cycle.
The first phase occurs early, while the black hole consumes material at an extreme rate near or above the Eddington limit—the point where outward radiation pressure matches the inward pull of gravity. Radio outflows at this stage are primarily interpreted as winds driven by super-Eddington accretion.
The second phase arrives hundreds or thousands of days later. At this delayed stage, the feeding rate drops significantly. Every well-constrained delayed event in the study aligned with launching when the feeding rate fell to about 2% of the black hole’s Eddington limit. This 2% threshold is already known to trigger jet formation in much smaller stellar-mass black holes residing within our own galaxy.
What the Universal Threshold Means for Future Astronomy
Establishing that supermassive black holes and stellar-mass systems share this critical trigger demonstrates that the fundamental physics governing black hole jets remains invariant across a mass range spanning roughly seven orders of magnitude.
Beyond theoretical implications, the discovery offers practical advantages for scheduling astronomy resources. By using the two percent feeding rule to anticipate when a black hole is likely to erupt with a delayed jet, researchers can target their observation windows more efficiently and avoid monitoring periods marked by inactivity.