A Star’s Spin May Explain Why Some Black Hole Flares Fade With Every Encounter
At the heart of nearly every large galaxy lurks a supermassive black hole, an object weighing millions or even billions of times more than the Sun. Although black holes are invisible by themselves, their violent interactions with nearby matter can produce some of the brightest and most dramatic flashes in the universe. When a star passes too close, the black hole’s tidal forces—the difference between the gravitational pull on the star’s near and far sides—can stretch, distort and ultimately destroy it. In some rare cases, however, the star is not completely torn apart. Its surviving core remains bound to the black hole, returning again and again for new close encounters. Each passage strips away more material and can trigger another flare, creating a cosmic spectacle that allows astronomers to observe the same stellar victim repeatedly.
These events, known as repeating partial tidal disruption events, or rpTDEs, are among the most valuable laboratories for studying the extreme physics around supermassive black holes. Ordinary tidal disruption events generally happen only once: the star is destroyed, and the resulting debris spirals inward, releasing energy as light over days, months or sometimes years. In an rpTDE, by contrast, the star’s dense core survives the first encounter and continues along an eccentric orbit. Every few months or several years, it swings back through the black hole’s strongest tidal field. During each passage, another layer of its atmosphere can be removed. Wide-field time-domain surveys, which repeatedly photograph enormous regions of the sky, have now identified roughly 10 such repeating systems. Their repeated flares offer an unprecedented opportunity to track how a star changes as it is gradually dismantled by a black hole.
Yet several of these systems have presented astronomers with a stubborn mystery. In at least four cases, the flares have become progressively dimmer with each successive encounter. At first glance, the explanation seems straightforward: if the star loses less mass every time it passes the black hole, then less debris should fall inward and the resulting flare should weaken. But earlier hydrodynamical simulations produced a different result. Those calculations showed that although the total amount of stripped material declined, the debris returned to the black hole more rapidly. The shorter fallback time compensated for the reduced mass, maintaining a similar peak rate of accretion and therefore a flare of roughly constant brightness. For researchers, the apparent mismatch between theory and observation remained unresolved for years.
A new study led by doctoral student Ananya Bandopadhyay of Syracuse University proposes that the missing factor is stellar rotation. The work, carried out with postdoctoral researcher Benjamin Amend, associate professor Eric Coughlin and collaborators at other institutions, shows that a star’s spin before its first encounter can strongly influence the evolution of an rpTDE. The researchers used hydrodynamical models to follow the star as it passes through the black hole’s tidal field, loses mass and changes its rotation. Their calculations indicate that a star arriving with rapid initial spin responds differently from a slowly rotating star. This previously overlooked initial condition can determine whether the debris fallback rate remains stable or decreases from one passage to the next, directly shaping the changing brightness of the observed flares.
The underlying physics involves a transfer of angular momentum. As the black hole’s gravity pulls unevenly across the star, the tidal force does more than remove gas from its outer layers. It also exerts a torque, accelerating the star’s rotation during each close passage. A slowly rotating star can therefore be spun up substantially. That additional rotation changes the structure and motion of the material stripped from the star, causing the debris to return toward the black hole on a shorter timescale. Even if the mass loss becomes smaller, the debris arrives more quickly, helping preserve the peak fallback rate and keeping the predicted flare nearly as bright. This mechanism explains why earlier simulations did not produce the steady dimming seen in several real systems: they did not fully account for how the star’s initial rotation could alter the amount of spin-up available during subsequent encounters.
According to the new simulations, a rapidly rotating star begins with much less room to be accelerated by the black hole’s tidal torque. Because it is already spinning quickly, each encounter produces comparatively little additional rotation. The fallback timescale for the material removed from the star consequently remains more nearly constant. As the star evolves and its increasingly concentrated core loses smaller amounts of mass, the quantity of debris returning to the black hole falls without being offset by a faster return. The peak fallback rate therefore declines, and so does the flare’s expected brightness. In this scenario, progressively dimmer flares are not an unexplained anomaly but a direct signature of the star’s rotational state before it ever met the black hole.
The proposed explanation may also reveal how these unusual star-black-hole systems formed. It is difficult to place an ordinary star on an orbit lasting only months around a supermassive black hole. A possible solution is the Hills mechanism, in which a tightly bound binary star system ventures too close to the black hole. The black hole’s tidal field breaks the binary apart, ejecting one star at enormous speed while capturing the other into a close, elongated orbit. This process naturally creates the short orbital periods observed in some rpTDEs. It may also explain why the captured star could have been rotating rapidly from the beginning. In a close binary, each star can become tidally locked, rotating once for every orbit around its companion. The tighter the binary, the faster that synchronized rotation becomes. A binary compact enough to produce a short-period captured star would therefore leave behind a rapidly spinning survivor—precisely the type of star required by the new model.
The result links two otherwise puzzling features of rpTDEs: their unusually tight orbits and their fading flares. Both may be consequences of the same violent event—the disruption of a binary system and the capture of one of its members by a supermassive black hole. The finding also gives astronomers a new way to interpret time-series observations. The brightness of each flare may encode information not only about how much mass the star has lost, but also about its internal structure, its rotation and the circumstances of its capture. Low-mass stars, whose material is less centrally concentrated, may become increasingly vulnerable as their outer layers are removed. More massive stars, with dense cores and layered internal structures, may lose progressively thinner envelopes while preserving their central regions. Rotation adds another dimension to this picture, potentially explaining why systems with apparently similar mass-loss histories can display very different patterns of brightness.
The implications may extend beyond rare repeating tidal disruption events. Coughlin and his colleagues suggest that the Hills mechanism could also have produced some of the young, fast-moving stars found near Sagittarius A*, the supermassive black hole at the center of the Milky Way. Those stars may be survivors of disrupted binaries, flung across the galaxy or captured into unusual orbits by the same gravitational process. By connecting stellar spin, binary disruption and fading black hole flares, the Syracuse study offers a broader framework for understanding how stars move through the crowded and hazardous environments surrounding supermassive black holes. Future surveys capable of detecting more repeating events, measuring their flare profiles across different wavelengths and following them for many years may test whether rapid initial rotation consistently predicts a decline in brightness. If it does, the fading light from a repeatedly wounded star could become a powerful record of a black hole’s past—and of the stellar system that sent the star there.
Subject of Research: Repeating partial tidal disruption events, stellar rotation, supermassive black holes, and the Hills mechanism.
Web References: The Astrophysical Journal study; Eric Coughlin, Syracuse University; Syracuse University Department of Physics.
References: The Astrophysical Journal; related previous study: https://iopscience.iop.org/article/10.3847/1538-4357/ad6a5a.
Image Credits: NASA / S. Gezari (Johns Hopkins University) / J. Guillochon (University of California, Santa Cruz).
Keywords
Supermassive black holes, tidal disruption events, repeating partial tidal disruption events, stellar rotation, stellar spin, hydrodynamical simulations, Hills mechanism, binary stars, stellar debris, accretion flares, Sagittarius A*.
