Astronomers have uncovered a universal rule governing when supermassive and stellar black holes launch powerful particle jets into space. By studying 20 tidal disruption events, researchers determined that these cosmic burps occur at a critical feeding threshold when accretion drops to 2 percent of the Eddington limit.
Black holes are famous for pulling in everything around them, but they are also actually very messy eaters,
according to Dr Adelle Goodwin, an astrophysicist at Curtin University in Western Australia and a Forrest Research Foundation fellow. When a passing star wanders too close to a supermassive black hole, intense gravitational forces subject the star to a destructive stretching process known as spaghettification. Only about half of the shredded stellar material is eventually swallowed, while the rest is violently expelled into space in luminous, high-speed particle streams.
Solving a Long-Standing Cosmic Puzzle
While astrophysicists have long observed these massive outflows, the timing of the eruptions remained deeply puzzling. Some black holes fire radio jets almost immediately after devouring a star, while others stay dormant for months or even years before suddenly erupting. An international research team co-led by Andrew Mummery of the Institute for Advanced Study and Goodwin set out to decode this erratic behavior.
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 fire up their jets months or even years later suddenly?
Andrew Mummery, Institute for Advanced Study
The breakthrough emerged from discussions at an astrophysics conference in Madrid, where Mummery and Goodwin considered whether physical laws governing tiny black holes might also apply to galactic giants. By cross-checking data, the team confirmed their hypothesis, publishing their findings in Nature Astronomy.
Observing Tidal Disruption Events Across Wavelengths
Because supermassive black holes evolve over millennia, watching them feed in real-time is normally impossible. To bypass this limitation, researchers monitored 20 tidal disruption events, rare phenomena where supermassive black holes tear apart wandering stars and compress an otherwise glacial feeding process into a window of just a few years.
The team combined observations spanning optical, ultraviolet, X-ray, and radio wavelengths. Goodwin noted that radio is the only frequency that allows scientists to watch jets and outflows as they physically move outward into space.
The Universal Two-Percent Rule
Through this multi-wavelength tracking, the researchers established that black holes launch jets in two distinct phases of their feeding cycles. The first occurs early, when the black hole consumes matter at an extremely high rate. The second phase happens hundreds or thousands of days later, once the accretion rate slows down.
Specifically, the delayed jets erupt when feeding drops to approximately 2 percent of the maximum rate at which a black hole can swallow material, known as the Eddington limit. This identical 2 percent threshold triggers jet formation in stellar-mass black holes inside the Milky Way, which weigh roughly 10 to 50 times the mass of the Sun. Despite supermassive black holes being millions or billions of times heavier, both classes obey the exact same physical law.
Optimizing Future Telescope Observations
Dr Sara Webb, an astrophysicist at Swinburne University who was not involved in the project, noted that the research proves supermassive black holes behave rather predictably at specific evolutionary points.
By establishing predictable timelines, the discovery allows astronomers to narrow observation windows and maximize efficiency for high-demand facilities, including the soon-to-be-completed Square Kilometre Array radio telescope in Western Australia. Researchers can now forecast not just when jets will emerge, but also evaluate their strength and dependence on underlying black hole properties.
