Black hole jets switch on at 2% of maximum feeding — every size, same rule

Something switches on inside a black hole when its feeding rate drops to exactly 2% of maximum. At that precise fraction — identical whether the black hole is ten times the mass of the Sun or a billion times heavier — a jet of plasma ignites and begins accelerating toward the speed of light.

For decades, astronomers have watched these jets tear through galaxies without understanding what launched them. The jets are among the most energetic phenomena in the universe, extending millions of light-years and carrying enough energy to reshape the gas clouds from which new stars form. The trigger was unknown. A study in Nature Astronomy, analyzing twenty events where stars were torn apart and consumed by black holes, has found it.

The answer is 2% of the Eddington limit — the theoretical feeding rate at which a black hole’s outward radiation pressure exactly balances its inward gravitational pull. When the accretion rate drops below that threshold, the jet fires. Every time. At every scale.

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How they found the trigger

Dr. Adelle Goodwin at Curtin University’s International Centre for Radio Astronomy Research and Dr. Andrew Mummery at the Institute for Advanced Study in Princeton spent years assembling observations of tidal disruption events — the rare, violent occurrences where a star wanders too close to a supermassive black hole and is ripped apart by gravitational tides. The event compresses what would normally be millions of years of black hole feeding into a burst lasting months to years.

That compression is what makes tidal disruption events such a precise laboratory. Instead of waiting for a normal feeding episode in an active galactic nucleus — which can persist for millions of years — researchers can watch an entire accretion cycle from start to finish. The team gathered optical, ultraviolet, X-ray, and radio observations from telescopes across four continents and in space, tracking the feeding rate as each black hole consumed the disrupted star.

Of the twenty events in their sample, ten had sufficient data quality to measure the accretion rate at the moment a jet appeared. In each case, the measurement fell at or very near 2% of the Eddington limit. The black holes ranged across twelve orders of magnitude in mass — from a few stellar masses to hundreds of millions of solar masses — yet the threshold held in every instance.

The 2% rule and the Eddington limit

The Eddington limit, named for the British astrophysicist Arthur Eddington, describes the feeding rate at which radiation pressure on infalling material exactly balances gravity. Above the Eddington rate, the outgoing radiation blows the fuel supply away; below it, matter falls inward more freely. For black holes accreting at very high rates, the physics is dominated by radiation; at low rates, the geometry of the accretion disc itself changes.

The 2% figure sits in a regime known as the hard state, where the accretion disc transitions from a geometrically thin, optically thick structure to a puffier, hotter, less efficient geometry. This structural transition was already known from studies of X-ray binaries — small systems where a stellar-mass black hole feeds from a companion star — but whether it triggered jets in the same way across supermassive systems was unverified.

The new result says yes. The mechanism that launches jets does not respond to the absolute mass of the black hole. It responds only to the ratio of accretion rate to Eddington limit. A stellar-mass black hole feeding at 2% of its Eddington rate and a supermassive black hole doing the same produce jets via identical physics, separated by twelve orders of magnitude in physical scale. The universality suggests that jet physics is fundamentally scale-invariant — governed by dimensionless ratios rather than absolute quantities.

What it does not explain

Ten events is a small statistical sample, and the researchers are careful about the limits of their conclusion. The accretion rate during a tidal disruption event is not directly measurable — it is estimated from the optical, ultraviolet, and X-ray luminosity of the event, which introduces uncertainties. The conversion from observed brightness to actual feeding rate depends on models of disc emission that are still debated.

More fundamentally, knowing when the jet fires is not the same as knowing how. The 2% threshold identifies the trigger point with precision, but the physical mechanism that converts infalling matter into a relativistic jet at that exact moment remains an open problem. Leading candidates involve the magnetic field structure of the disc and the rotation of the black hole — the Blandford–Znajek mechanism — but the new data cannot distinguish between competing models.

There is also the question of jets that do not fire. Not every tidal disruption event produces a jet even when accretion rates cross the 2% threshold. The geometry of the black hole’s spin axis and the configuration of magnetic flux near the event horizon may determine whether a jet actually forms, or whether the energy is dissipated by other means.

The moment in Madrid

The breakthrough did not emerge from a computer in Australia or New Jersey. It happened in a bar near a conference site in Madrid, where Goodwin and Mummery compared notes over drinks. They had each been analyzing the tidal disruption data independently and realized, looking at the numbers together, that the threshold matched across every event in both their samples.

The team then went back and confirmed the finding systematically against the full dataset. Mummery described the result: “These black holes are separated by enormous differences in mass, but they appear to switch on their jets at the same point in their feeding process.”

The result imposes a tight new constraint on theories of relativistic jet formation. Theoretical models that cannot reproduce a universal 2% trigger across all black hole masses are now in tension with the data.

Common questions about black hole jets

What is a tidal disruption event?

A tidal disruption event occurs when a star passes close enough to a black hole that gravitational tidal forces — stronger on the near side of the star than the far side — exceed the star’s own self-gravity and tear it apart. The disrupted stellar material forms an accretion disc and is consumed over months to years. These events are detectable across the observable universe as bright transient flashes.

What is the Eddington limit?

The Eddington limit is the feeding rate at which the outward radiation pressure from accreting material exactly balances the inward gravitational force. For a ten-solar-mass black hole, the Eddington rate is roughly a hundredth of a solar mass per year; for a supermassive black hole of a billion solar masses, it is roughly ten solar masses per year. The 2% threshold in this study is 2% of whichever Eddington rate applies to a given black hole.

Why does the jet threshold matter for galaxy evolution?

Jets from supermassive black holes are one of the primary mechanisms by which black holes regulate star formation in their host galaxies. When a jet fires, it injects energy into the surrounding gas, heating it and suppressing new star formation. Understanding precisely when jets switch on — and what controls that switch — is essential for accurate models of how galaxies evolve over cosmic time. The 2% rule gives theorists a precise, observationally grounded constraint to work from.

How will future telescopes test this?

NASA’s Nancy Grace Roman Space Telescope is designed in part to detect thousands of tidal disruption events across its wide-field survey, vastly extending the current sample of ten reliable events. The team intends to determine whether black hole spin, host galaxy type, or the magnetic field configuration at disruption time modifies the threshold — or whether 2% proves genuinely universal across the full population of black holes.

NASA’s Nancy Grace Roman Space Telescope, which launched in late August 2026 and now carries enough fuel for at least 22 years of operations, is expected to detect thousands of tidal disruption events in its first years of wide-field survey work — vastly extending the current sample of ten reliable events. Whether 2% proves universal across that broader population, or breaks down for black holes at the extremes of mass and spin, will determine whether this number enters the textbooks as a fundamental constant of jet physics or remains an approximation holding for one particular class of feeding events.

The paper was led by Goodwin and Mummery, with co-investigators drawn from observatories in Australia, the United States, India, South Africa, and the United Kingdom.

Reference: Goodwin, Mummery et al., “A universal critical accretion rate for black hole jet formation,” Nature Astronomy, 2026. DOI: 10.1038/s41550-026-02951-1

Tags: astronomy, black holes, Nature Astronomy, Curtin University, Eddington limit, relativistic jets

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