Black Holes of Every Size Follow the Same Rule for Launching Jets, Study Finds

Astronomers have identified what they describe as a universal rule governing when black holes fire off jets of high-speed particles, regardless of whether the black hole weighs a few times as much as the sun or several million times as much. The finding, published this month in Nature Astronomy, suggests that black holes across a vast range of sizes switch on their jets at the same critical point in their feeding cycle.

The study, led by Andrew Mummery of the Institute for Advanced Study in Princeton and Adelle Goodwin of Curtin University's International Centre for Radio Astronomy Research in Australia, analyzed 20 tidal disruption events — the rare instances in which a supermassive black hole shreds a star that has wandered too close. Ten of those events had data of sufficient quality to model precisely, allowing the researchers to compare the timing of each black hole's radio jets against how quickly it was consuming the shredded stellar debris.

A Feeding Threshold, Not a Fixed Delay

The researchers found that jets switch on when a black hole's accretion rate — how fast it is pulling in surrounding material — drops to roughly 2% of the Eddington limit, the theoretical maximum rate at which a black hole can feed before its own radiation blows material away. Some supermassive black holes reached that threshold within days of disrupting a star and lit up almost immediately; others took hundreds or more than a thousand days to slow down enough to cross it, producing jets that appeared to switch on suddenly long after the initial stellar destruction had faded from view.

That same 2% threshold has previously been observed triggering jets from much smaller, stellar-mass black holes closer to home in the Milky Way, which are fed by a companion star rather than a one-time stellar meal. "Why do some supermassive black holes blast out radio jets right after shredding a star, while others just sit there looking completely dormant?" Mummery said of the puzzle that motivated the research, noting that the answer turned out to depend on feeding rate rather than the black hole's size or the details of any individual disruption.

Built From Years of Multi-Telescope Observing

Confirming the pattern required years of coordinated observations using optical, ultraviolet, X-ray and radio telescopes in the United States, Australia, India and South Africa, since a single tidal disruption event can take months or years to run its full course, and NASA missions built to study transient events such as tidal disruptions have generated much of the underlying X-ray and ultraviolet data that made the comparison possible, according to background material from NASA's description of how these stellar-shredding events are studied. Goodwin's institution, Curtin University, credited the collaboration with resolving a question that had puzzled astronomers since the first jetted tidal disruption events were discovered more than a decade ago.

The paper's authors say the universal threshold gives astronomers a predictive tool: knowing a black hole's feeding rate should now make it possible to forecast whether and roughly when a jet will appear, rather than waiting to catch one by chance. That predictive power will matter more as new radio facilities come online later this decade. The Square Kilometre Array, expected to begin science operations in 2028, is designed to catch far more tidal disruption events in progress, and the research team said the accretion-rate rule identified in the current study, available via its journal publication, should help those future surveys know when and where to look for delayed jets rather than relying on repeated blind monitoring of every disruption they detect.