Scientists pinpoint when black holes 'burp' jets after devouring stars

A star drifts too close to a supermassive black hole, and gravity does what gravity does—it tears the star apart. Astronomers call this a tidal disruption event, and it is one of the most violent phenomena in the universe. Some of the star's material falls into the black hole's maw. The rest gets hurled back out into space in jets of staggering power, sometimes within days of the destruction, sometimes not until months or years have passed. For years, this timing puzzle confounded researchers: why did some black holes fire jets almost immediately after consuming a star, while others seemed to wait in silence before suddenly switching on, with no obvious trigger?

Adelle Goodwin of Curtin University and Andrew Mummery of Princeton's Institute for Advanced Study set out to solve that puzzle. They examined twenty tidal disruption events, using optical, ultraviolet, X-ray, and radio observations to build a complete picture. From that sample, they narrowed their focus to ten events where they could reliably measure both how fast the black hole was feeding and when its radio jets appeared. What they found was a pattern hidden in plain sight: two distinct windows when jets emerge. The first arrives early, while the black hole is still gorging at extreme rates in the immediate aftermath of the star's destruction. The second comes much later—sometimes hundreds or thousands of days afterward—once the black hole's feeding rate has slowed to roughly two percent of what astronomers call the Eddington limit, the theoretical point where outward radiation pressure exactly balances the black hole's inward pull of gravity.

Goodwin described the phenomenon with a metaphor that stuck: a black hole burp. When a black hole tears apart a star, it does not consume everything in an orderly fashion. Some material vanishes into the event horizon. Some gets ejected in powerful jets and outflows that can travel across enormous distances and reshape the galaxies around them. The burp metaphor captures something true about the process—it is messy, it is delayed, it is a kind of cosmic belch that happens on its own schedule.

The key to cracking the puzzle was radio observation. While optical telescopes can track the initial bright flare from a tidal disruption event, that light fades relatively quickly. Radio telescopes, by contrast, can trace the outflowing material long after the visible fireworks have dimmed. By tracking radio emissions over time, Goodwin and Mummery could see the delayed second phase of jet activity—something that had been nearly invisible when researchers studied individual events in isolation. The pattern emerged only when they compared multiple cases side by side. In every instance, the late jets switched on once the black hole's feeding rate crossed that same two-percent threshold. It was not arbitrary timing. It was physics.

What made the finding even more striking was its universality. The two-percent Eddington limit had already been known to trigger jets in stellar-mass black holes—small objects, typically only about ten times the mass of the Sun, scattered throughout our own galaxy. Finding the same threshold at work in supermassive black holes, millions of times heavier, suggested something fundamental about how black holes work. The physics of jet formation, it seemed, did not care about size. A black hole ten times the Sun's mass and a black hole millions of times heavier would both switch on their jets at the same point in their feeding process. That uniformity pointed to something deep in the nature of black holes themselves—a universal rule that governs their behavior regardless of their mass.