Black holes do not appear to follow a fixed countdown before producing a delayed jet. A 2026 Nature Astronomy study by Adelle J. Goodwin and Andrew Mummery finds that a second outflow can begin when the feeding rate falls to about 0.02 of the Eddington limit. In tidal-disruption events, that change can occur hundreds or thousands of days after a star is torn apart, which explains the apparent wait.
The delay is caused by an accretion-state change
The Eddington limit is the approximate feeding rate at which radiation pressure from matter falling inward balances the black hole’s gravity. The study identifies a critical transition near 2% of that limit. When the debris from a destroyed star declines to this rate, the black hole can enter a different accretion state and launch a late radio-emitting outflow.
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That means “thousands of days” is a timescale produced by the evolution of the debris, not a universal timer that starts at the moment of disruption. A system whose debris settles quickly can reach the transition sooner; another can take much longer.
The result addresses a long-standing observational puzzle. As Andrew Mummery of the Institute for Advanced Study put it, “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?”
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Why tidal-disruption events make the timing measurable
Supermassive black holes normally evolve too slowly for astronomers to watch an entire feeding cycle. Their characteristic changes can take thousands of years. A tidal-disruption event, or TDE, creates a much faster experiment: a star passes close enough to be torn apart, and some of its debris forms a temporary accretion flow that can be monitored over several years.
Goodwin and Mummery examined 20 TDEs. They selected 10 events with sufficiently strong multi-wavelength observations to model both the changing feeding rate and the timing of radio outflows. Comparing those measurements allowed the team to connect a late outflow with the point at which the accretion rate had declined to the critical level.
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Adelle Goodwin, of Curtin University and the International Centre for Radio Astronomy Research, described the process this way: “When a black hole tears apart a star, it does not swallow everything neatly.” The disrupted material follows a changing, uneven path toward the black hole, so the supply rate and the resulting outflow can evolve in stages.
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Two physically different outflow phases
| Feature | Early phase | Delayed phase |
|---|---|---|
| Accretion regime | Super-Eddington feeding, above the balance rate | Feeding has declined to approximately 0.02 of the Eddington limit |
| Timing after disruption | Prompt, during the initial high-rate episode | Hundreds to thousands of days later in the observed TDEs |
| Observable signature | An outflow that can appear soon after the star is shredded | A later radio outflow after an apparently quiet interval |
| Physical interpretation | Massive, high-rate accretion drives an early outflow | A transition to a lower-rate accretion state enables a distinct jet |
The two signals should therefore not be treated as one jet that simply fades and restarts. They represent different stages of the accretion flow. Some TDEs show the early phase without an obvious late event; others may look dormant at first and become radio-bright only after the state transition.
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What the 2% threshold reveals about black-hole physics
The most significant result is that the same approximate transition point is known from observations of stellar-mass black holes. Although a stellar black hole and a supermassive black hole differ enormously in mass and physical size, both can switch their outflow behavior when their feeding rate reaches a similar fraction of the Eddington value.
This agreement supports a scale-invariant picture of accretion and jet launching. The governing process appears to depend more on the dimensionless feeding rate than on the black hole’s absolute mass. A threshold expressed as a fraction of Eddington can therefore provide a common framework for interpreting outflows across mass regimes.
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Why the finding changes observing strategy
Radio observations are expensive in telescope time, and a source that is quiet immediately after a disruption may not be an unpromising target. If its debris is still declining toward the critical rate, a later campaign could catch the delayed outflow.
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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteThe threshold gives astronomers a way to estimate when a source may become interesting again by tracking the feeding-rate decline rather than counting a standard number of days. A 2026 Space.com report described a two-to-six-year follow-up window for late radio signals as an observing recommendation. That window is not a universal prediction from the study: the actual delay depends on each event’s debris dynamics.
More reliable timing forecasts could help observatories schedule radio and other multi-wavelength resources around the period when a state transition is most likely. The framework is predictive in the sense that it links the expected outflow to a measurable accretion rate, not because it assigns every TDE the same launch date.
What the study does not establish
- It does not show that every supermassive black hole will launch a jet at exactly 2% of Eddington.
- It does not convert the threshold into a fixed number of elapsed days for every tidal-disruption event.
- The late signal is described as a physically distinct outflow, but the data set does not imply that every event must display both phases equally clearly.
- The sample contains 20 analyzed TDEs, with detailed modeling for 10 high-quality events, so additional events will test how broadly the pattern applies.
The strongest conclusion is therefore conditional: when a TDE’s feeding rate declines through the critical regime, a delayed outflow becomes physically expected. The elapsed time remains event-dependent because the disrupted star’s material does not settle in an identical way each time.
Why this matters beyond one class of stellar explosions
TDEs provide a rare time-lapse view of supermassive-black-hole feeding. By compressing a process that normally unfolds over millennia into a few observable years, they let astronomers compare the behavior of giant black holes with the better-monitored state changes of stellar-mass systems.
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If future observations confirm the same transition across more events, astronomers could use the dimensionless accretion rate as a practical trigger for jet searches. That would improve models of how black holes transfer energy and magnetic fields into their surroundings and clarify why apparently dormant objects can later produce powerful radio outflows.
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