Black Holes Large and Small Launch Jets at the Same Universal Threshold

For decades, astronomers have wondered whether the violent jets that blast away from black holes—whether a star-sized corpse in our own galaxy or a supermassive monster anchoring a distant galaxy—are governed by the same underlying physics. A new study published in Nature Astronomy provides the most compelling evidence yet that they are. By studying tidal disruption events, cataclysmic moments when a star is shredded by a supermassive black hole, researchers Adelle J. Goodwin of the International Centre for Radio Astronomy Research at Curtin University and Andrew Mummery of the Institute for Advanced Study have shown that black holes of vastly different masses launch outflows at the same critical accretion rate, a finding that points to a single, scale-invariant process governing jet formation across the universe.

The question of universality has been difficult to answer because of the sheer disparity of timescales involved. Stellar-mass black holes, which weigh in at a few to a few dozen times the mass of the sun, evolve quickly: their accretion disks brighten and fade over weeks and months, and astronomers have watched them undergo state transitions many times. During these outbursts, the accretion flow switches between a hot, radiatively inefficient mode and a cooler, geometrically thin disk, and it is precisely at these transitions that jets are switched on or off. Decades of monitoring have pinned down the critical accretion rate at which this happens with considerable precision.

Supermassive black holes, by contrast, are far less cooperative. The accretion flows that power active galactic nuclei evolve over thousands of years, which means that no single observer—or even several generations of observers—can watch one system cross the critical threshold where jets are born. As a result, whether the well-established rules of stellar-mass black holes apply to their supermassive cousins remained an open and contested question, one with profound implications for how galaxies grow and how black holes shape their cosmic environments.

Goodwin and Mummery realized that tidal disruption events offered a way around this bottleneck. When a star wanders too close to a supermassive black hole, tidal forces rip it apart, and its debris forms a glowing accretion disk around the hole. Unlike ordinary active galactic nuclei, these newly formed disks evolve dramatically on timescales of just years, blazing bright at super-Eddington accretion rates before fading steadily. That means a single tidal disruption event can carry a black hole through the entire range of accretion rates where jets are expected to appear, all within a human lifetime.

The researchers assembled multiwavelength data for a sample of tidal disruption events, combining radio observations—which trace outflows launched by the black hole—with detailed modeling of the evolving accretion disk using publicly available fitting tools. The disk modeling allowed them to reconstruct the accretion rate of each system at any moment in its evolution, while the radio data, interpreted through synchrotron spectral fitting, revealed when outflows were launched and how fast they were moving. By cross-referencing the two, they could determine the accretion rate at the precise moment each outflow was ejected.

The results were striking. Every tidal disruption event in the sample launched an early outflow while accreting at super-Eddington rates, consistent with the powerful disk winds expected when radiation pressure overwhelms gravity. But many systems also produced a second, physically distinct outflow later in their evolution, and these delayed ejections occurred when the disk accretion rate had fallen to a critical value of roughly two percent of the Eddington luminosity—the same threshold, scaled by mass, at which stellar-mass black holes are known to undergo their hard-to-soft state transitions and launch their characteristic compact jets. In one well-studied event, ASASSN-14li, the accretion rate at the time of outflow launch could be constrained with particular precision, and it matched the universal value.

This convergence is remarkable because the two classes of systems differ by factors of a million or more in black hole mass. The Eddington rate itself scales with mass, so expressing the accretion rate as a fraction of the Eddington value is the natural way to compare systems across cosmic scales. That the critical fraction comes out the same—about two percent—for both stellar-mass black holes and supermassive black holes strongly suggests that the physical mechanism responsible for jet launching does not care about scale. The processes are thought to involve the geometry of the inner accretion flow, where a hot, tenuous, magnetized plasma can collimate magnetic fields into a jet, and the new results indicate that the conditions for this configuration arise at the same fractional accretion rate regardless of the black hole’s mass.

The framework also resolves a puzzle that has dogged tidal disruption observers for years: why some events show prompt radio emission, why others flare in radio years after the optical outburst, and why surveys now routinely detect late-time radio re-brightenings that earlier models struggled to explain. In the new picture, the prompt outflows are super-Eddington winds ejected when the disk first forms, while delayed radio flares mark the launch of a second, jet-like outflow as the fading disk crosses the universal critical threshold. The timing of the delayed flares should then depend on how quickly each disk evolves from its super-Eddington peak down to two percent of Eddington, a prediction the authors tested with a modeled population of tidal disruption disks and found consistent with the observed distribution of outflow launch times.

Beyond tidally disrupted stars, the findings extend naturally to the broader population of accreting black holes. The critical threshold provides a predictive tool: given a black hole’s mass and its current accretion rate, astronomers can now estimate whether jets should be on or off, and when a fading system will cross the line. For active galactic nuclei, whose long-term evolution is normally inaccessible, this offers a way to infer jet activity from snapshot measurements of accretion rate. It also connects to long-standing empirical relations, such as the fundamental plane of black hole activity that links radio and X-ray luminosities across mass scales, which hinted at scale-invariant physics but lacked the direct observational anchor that tidal disruption events now supply.

The implications reach beyond pure astrophysics. Jets launched by supermassive black holes inject enormous amounts of energy into their host galaxies, heating gas, suppressing star formation, and shaping the largest structures in the universe. Understanding exactly when and why these jets switch on is therefore essential for modeling galaxy evolution itself. If a single scale-invariant threshold controls jet formation everywhere, simulations of galaxy feedback can be anchored to a universal rule rather than calibrated separately for each mass regime. And with new time-domain surveys set to discover tidal disruption events by the thousands in the coming years, the framework developed by Goodwin and Mummery promises a growing catalog of natural experiments—each a black hole caught mid-transformation, its jet poised to switch on at the same universal moment.

Subject of Research: The universal critical accretion rate governing jet formation in black holes across mass scales, probed through tidal disruption events

Article Title: A universal critical accretion rate for black hole jet formation

Article References: A universal critical accretion rate for black hole jet formation. (n.d.). https://doi.org/10.1038/s41550-026-02951-1

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02951-1

Keywords: black holes, jets, tidal disruption events, accretion, supermassive black holes, Eddington limit, radio astronomy, active galactic nuclei, outflows, state transitions, accretion disks, Nature Astronomy