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Black holes of radically different sizes appear to launch powerful jets when their feeding rate falls to roughly 2% of the Eddington limit. The scale-invariant threshold, reported by an international team on September 17, 2026, may let astronomers forecast delayed radio eruptions months or years after a star is destroyed.
The universal rule: jets switch on near 2% of Eddington
The study, published in Nature Astronomy as “A universal critical accretion rate for black hole jet formation,” identifies a common trigger across black holes ranging from stellar-mass objects of about 10 Suns to supermassive black holes millions of times heavier.
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That trigger is an accretion rate—the rate at which matter feeds the black hole—of approximately 2% of the Eddington limit. The Eddington limit is the point at which outward radiation pressure from infalling matter balances the black hole’s gravitational pull. In this picture, the absolute amount of matter differs enormously between systems, but the critical fraction of the system’s own Eddington rate remains similar.
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How a shredded star reveals the timing
The team studied tidal-disruption events, or TDEs. During a TDE, a supermassive black hole’s gravity tears apart a passing star. Debris falls back over a comparatively short, observable period, compressing a feeding episode that would normally evolve over thousands or millions of years into a sequence that astronomers can monitor over years.
Researchers combined optical, ultraviolet, X-ray and radio observations for 20 TDEs. They selected 10 high-quality events for dependable modeling of the feeding rate and the timing of radio outflows. That multi-wavelength approach links the changing supply of debris to the moment a jet appears.
Two jet-launching phases
| Phase | Timing after disruption | Feeding rate | Observational signature | Best observing strategy |
|---|---|---|---|---|
| Early phase | Shortly after the star is disrupted | Extreme, near the beginning of the fallback episode | A jet can appear promptly alongside the bright early flare | Begin radio and high-energy monitoring immediately and maintain broad multi-wavelength coverage |
| Delayed phase | Hundreds to thousands of days later | Falls to about 2% of the Eddington limit | Radio outflow turns on after the initial flare has faded | Track the modeled decline in feeding and schedule radio observations around the predicted threshold |
The delayed phase resolves a long-standing observational puzzle: some TDEs produce radio jets soon after the star is torn apart, while others look quiet before suddenly erupting months or years later. The new interpretation is that the systems are crossing the same critical accretion state at different times.
Why the 2% threshold matters
A common switch across mass scales
Small black holes in the Milky Way are already known to launch jets when their feeding rate reaches a similar fraction of the Eddington limit. Finding approximately the same threshold in supermassive black holes indicates that the underlying transition does not depend strongly on mass. A stellar black hole and a galaxy-center giant reach the switch at very different physical inflow rates, but at a comparable normalized rate.
A practical forecast for radio astronomy
Once astronomers estimate how quickly a TDE’s fallback rate is declining, they can look for the point at which it approaches 2% of Eddington. That forecast can guide requests for scarce radio-telescope time instead of relying on occasional follow-up observations after an eruption has already begun.
The Institute for Advanced Study says this approach could reduce unproductive monitoring and improve the chance of catching brief or rapidly evolving outbursts. The Square Kilometre Array, expected to begin collecting scientific data in 2028, is specifically cited as a facility that could benefit from predicted observing windows.
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What remains uncertain
The result is a strong pattern, not a guarantee that every black hole will produce an observable jet at exactly the same moment. The modeling relied on 10 high-quality TDEs from a 20-event sample, and radio detectability can depend on jet direction, surrounding gas and the sensitivity of a given facility. A system may therefore cross the critical feeding rate without producing an immediately obvious signal in every band.
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The study also distinguishes the timing rule from the detailed mechanism that shapes each eruption. Magnetic-field structure, the geometry of the accretion flow and the environment around the black hole can affect how efficiently an outflow forms and how bright it appears.
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Why these eruptions affect galaxies
Jets carry energy and particles far beyond the immediate accretion flow. As the Institute for Advanced Study describes them, these cosmic “burps” can blast material across staggering distances and influence the evolution of their host galaxies. Predicting when a jet begins gives astronomers a better chance to measure that energy transfer while it is happening, rather than inferring it from a fading remnant.
Andrew Mummery, a co-leader at the Institute for Advanced Study, said the team wanted to solve why some supermassive black holes “blast out radio jets right after shredding a star,” while others remain dormant before firing months or years later. Co-leader Adelle Goodwin of Curtin University’s International Centre for Radio Astronomy Research emphasized that a disrupted star is not swallowed neatly; its debris creates a changing, extended feeding process that can eventually reach the jet-launching threshold.
Frequently Asked Questions
What is the Eddington limit?
It is the accretion rate at which radiation pressure pushing outward balances a black hole’s gravity pulling matter inward. The reported jet transition occurs at about 2% of that system-specific limit.
How could astronomers predict a delayed jet?
They can model the declining fallback rate from optical, ultraviolet and X-ray observations, then target radio observations as the modeled rate approaches approximately 2% of Eddington.
Does every tidal-disruption event produce a jet?
No. The study identifies a common critical rate among the events it modeled, but jet visibility can also depend on flow geometry, magnetic fields, viewing angle and the surrounding environment.
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