Black holes, when size doesn't matter

The story of a star that ends up too close to a black hole may hold surprises even after its destruction. Sometimes the black hole expels matter into space almost immediately. Other times, however, when everything seems to have calmed down, it continues to speak for months, even years later: as if it still has a message to send. But why wait so long? The answer may depend on how quickly the black hole feeds. And surprisingly, the rule seems to be the same for black holes of very different masses. To find it, refer to a study published last week in Nature Astronomy and conducted by Adelle Goodwin of the International Centre for Radio Astronomy Research at Curtin University in Australia, and Andrew Mummery of the Institute for Advanced Study in Princeton, in the United States. The two researchers studied the events of tidal disruption, analyzing what happens to the stellar matter after the star's destruction. Some of the stellar debris remains gravitationally bound to the black hole and, instead of dispersing into space, ends up forming an accretion disk around it. Here the matter orbits, is heated, and emits radiation. Some of the material then falls beyond the event horizon, the boundary beyond which nothing can escape the black hole, while another part can be expelled into space in the form of winds or jets of matter. When these flows of matter meet the surrounding gas, they can generate shock waves that accelerate particles, producing radio emission. By studying how this emission changes over time, it is possible to trace back to the moment the matter was expelled. But to understand the conditions under which these flows are formed, it is also necessary to follow the evolution of the disk. In active galactic nuclei, at the center of which are supermassive black holes that accrete matter, the overall changes in the disk can take thousands of years: too long to directly observe the passage between different accretion states. Tidal disruption events, on the other hand, offer a special opportunity: the disk formed from the stellar debris is much more compact and evolves over a few years, allowing us to follow how the accretion rate changes over time, that is, the amount of matter that reaches the black hole in a unit of time. Goodwin and Mummery examined twenty tidal disruption events, using observations from radio to visible and ultraviolet light, up to X-rays. Among these, they selected the events with sufficiently complete data for analysis, arriving at eleven analyzed episodes. By studying the expansion of the regions from which the radio waves originate, the researchers estimated when the matter was expelled. They then compared the observations at other wavelengths with relativistic models of the accretion disk to reconstruct how the rate at which the black hole fed changed over time. Two distinct phases in which supermassive black holes can expel matter emerged from the analysis. The first occurs in the initial phases of the event, when the accretion rate is extremely high and can exceed the so-called Eddington accretion rate, related to the Eddington limit, which is the luminosity at which the outward push of radiation balances the gravitational attraction on the surrounding matter. In these conditions, the disk can produce powerful winds that carry some of the material outwards. The second phase occurs much later, when the black hole receives much less matter. This is where the interesting result emerges: the later flows are launched when the accretion rate drops to about two percent of the Eddington accretion rate. A far from random threshold: it coincides with that observed in stellar-mass black holes, which form from the collapse of massive stars and have masses generally between a few and several dozen times that of the Sun. In these systems, reaching the same threshold marks the transition to a different accretion state, associated with the formation of a compact and persistent radio jet. The new study suggests that a similar transition also occurs in supermassive black holes after a tidal disruption event. Unlike the flows produced in the initial phases of the event, the later flows would be associated with the formation of compact jets. Radio observations support this interpretation: while the emission associated with the immediate flows generally increases gradually, that of the later flows increases much more rapidly, compatible with a jet that continues to receive energy from the accretion disk. X-ray observations also provide clues supporting this interpretation. In some tidal disruption events, a late high-energy X-ray emission has been detected, compatible with the presence of a corona, a region of very hot plasma located near the black hole. Its appearance, sometimes coinciding with the new phase of radio activity, could indicate a change in the state of the accretion disk associated with the formation of the late jets. But why are these flows of matter not observed in all tidal disruption events? To seek an answer, Goodwin and Mummery simulated the evolution of a million events, considering stars and black holes with different characteristics. The simulations show that not all systems reach a sufficiently high accretion rate to produce the immediate flows, while others take many years before crossing the critical threshold associated with the later ones. This could explain why the immediate and later flows are only observed in a part of tidal disruption events. The researchers' predictions are in good agreement with the observations, both for the frequency of the flows and the times of their appearance. And despite the enormous mass differences, it seems that black holes follow the same rule when it comes to producing jets. As for the critical threshold, knowing it could help predict when a supermassive black hole will produce a new jet and schedule radio observations to catch its appearance. Even years after the destruction of a star, it is worth staying tuned: the black hole may still have something to say. To learn more: - Read the article on Nature Astronomy "A universal critical accretion rate for black hole jet formation" by Adelle J. Goodwin and Andrew Mummery Watch Gabriele Ghisellini's video on the different sizes of black holes: