Black Holes Ten Times the Sun's Mass and Black Holes Millions of Times Heavier Turn On Their Jets at Exactly the Same Point: When Feeding Drops to 2% of the Eddington Limit. Two Astronomers Worked It Out From 10 Shredded Stars, and the Pattern Clicked in a Bar in Madrid.

Black holes come in two wildly different weight classes, the stellar kind at around 10 times the mass of the Sun and the supermassive kind at millions or billions of solar masses, and astronomers have long suspected they obey the same physics despite the gap. Proving it has been the problem, because a supermassive black hole's feeding episode unfolds over thousands of years while a small one cycles in weeks. A study published in Nature Astronomy by Adelle Goodwin of Curtin University's International Centre for Radio Astronomy Research and Andrew Mummery of the Institute for Advanced Study in Princeton now reports that both launch their powerful jets at the same critical point: when the rate at which they swallow material drops to about 2 percent of the Eddington limit, the threshold at which outward radiation pressure balances gravity.

The trick was to find supermassive black holes on a human timescale. Goodwin and Mummery studied tidal disruption events, the rare cases in which a star wanders too close to a galaxy's central black hole and is ripped apart. The debris feeds the black hole in a burst that rises and fades over months to years rather than millennia, a compressed version of the whole cycle. The pair gathered optical, ultraviolet, X-ray and radio observations of 20 such events from telescopes in Australia, the United States, India, South Africa and space, and narrowed the sample to 10 in which they could reliably model both the black hole's feeding rate and the timing of its radio outflows.

The puzzle that started the work was timing. Some black holes blast out radio jets soon after shredding a star; others sit apparently dormant for months or years and then abruptly light up. "We were looking at these events and asking why the timing was so different," Goodwin said. "Then the pattern became clear. The delayed jets were appearing when the black hole's feeding rate dropped to the same critical point already known from much smaller black holes." That moment, she said, came not at a telescope but in a bar in Madrid during a conference, when she and Mummery realized the rule from stellar-mass systems fit the giants too.

The data show two distinct jet-launching phases. The first happens early, when the black hole is gorging at rates above the Eddington limit. The second arrives hundreds to thousands of days later, as the feeding rate falls through roughly 2 percent of Eddington, the same value that triggers jets during state transitions in X-ray binaries in the Milky Way. "These black holes are separated by enormous differences in mass, but they appear to switch on their jets at the same point in the feeding process," Goodwin said. "That tells us something fundamental about black holes: The physics does not seem to care how big they are."

Beyond the physics, the result is a scheduling tool. Radio telescope time is scarce and expensive, and astronomers chasing tidal disruption events have often pointed dishes at the wrong moment. "Knowing when to look is just as important as knowing where to look," Goodwin said. With the low-frequency half of the Square Kilometre Array being built in Western Australia at a total project cost above $2 billion, and next-generation surveys about to find far more shredded stars than anyone can follow, a physical signal for when the jet is coming means fewer wasted observations. Black holes, she said, are not neat vacuum cleaners but "messy eaters," and the study pins down when the burp comes.