Astrophysicists at Syracuse University have discovered that a star’s rapid rotation before its first encounter with a supermassive black hole explains why repeating partial tidal disruption event flares grow progressively dimmer over time, solving a two-year celestial mystery reported in The Astrophysical Journal.
Most galaxies harbor a supermassive black hole at their center, possessing gravitational environments strong enough to weigh millions or billions of times more than our sun. When a star ventures dangerously close to one of these enormous objects, destruction is not always immediate. Some stars survive the initial encounter and return for additional close passes, generating a fresh burst of light each time. These phenomena are known as repeating partial tidal disruption events, or rpTDEs.
Wide-field time-domain surveys make observing these interactions possible by repeatedly scanning large regions of the sky and tracking objects whose brightness changes. Yet astronomers have struggled to understand why roughly four out of the ten repeating systems identified so far produce flares that become steadily dimmer on each return.
How Black Holes Tear Apart Stars and Why Some Flares Keep Fading
In a standard tidal disruption event, the gravitational pull from a black hole varies so strongly across a nearby star that the star is completely torn apart. The resulting stellar debris begins falling toward, or accretes onto, the black hole. As that material loses energy, it releases light over periods ranging from days to months. Black holes themselves do not emit light, but a tidal disruption event temporarily supplies material that illuminates the surrounding region, giving astronomers an indirect way to investigate otherwise invisible objects.
Not every encounter ends with total destruction. If a star passes close to a black hole without crossing the threshold for total disruption, it loses only part of its mass, producing a partial tidal disruption event. The surviving core remains in orbit and returns for additional close encounters months or several years apart, shedding more material each time.
The amount of material stripped from a star depends heavily on its internal structure. A low-mass star behaves like a fluffy meringue, making it increasingly susceptible to tidal forces, whereas a higher-mass star features a concentrated, onion-like internal structure that sheds outer layers while keeping its dense core comparatively unchanged. These structural differences explain why repeating systems evolve differently, but they failed to explain why some flares mysteriously fade.
Previous hydrodynamical simulations revealed a major complication. Even when a star lost less material during each successive passage, the models predicted flares with approximately the same peak brightness. We were puzzled by this for two years,
Bandopadhyay says.
The Hidden Role of Stellar Torque and Rotation Rate
Previous research uncovered an important consequence of the black hole’s gravitational grip. Besides pulling material away from the star, tidal forces also apply torque, causing the star to rotate faster after each close encounter. That increased rotation changes how quickly stripped material returns toward the black hole. Even though less material comes back, it does so over a shorter period of time, and the more concentrated flow maintains a similar peak fallback rate and flare brightness.
To reproduce the fading flares that astronomers actually observe in the night sky, researchers needed a new ingredient
: a star that was already rotating rapidly before its first encounter with the black hole.
New simulations published in The Astrophysical Journal demonstrate that a rapidly spinning star cannot be spun up nearly as much during later passages. Without a large increase in rotation after each encounter, the time required for the stripped material to fall back toward the black hole remains relatively steady. As progressively less material is stripped from the star, the peak fallback rate finally decreases.
Tracing the Origin of Rapidly Spinning Surviving Stars
The same physical process that may trap these stars near supermassive black holes in the first place—namely, the breakup of a tight binary star system—could also explain why the stars were spinning so fast beforehand. Known repeating examples tracked by astronomers include systems designated as ASASSN-14ko, AT2018fyk, eRASSt-J045650, AT2022dbl, AT2020vdq, AT2021aeuk and AT2023uqm.
Research Team and Future Observation Outlook
The study was led by Syracuse University doctoral student Ananya Bandopadhyay, working alongside postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin in the Department of Physics, along with collaborators at other institutions.
By accounting for pre-encounter stellar spin, theoretical models now align much more closely with observational data from wide-field surveys, providing astrophysicists with a clearer framework to decode the complex gravitational dance between supermassive black holes and the resilient stars that repeatedly survive them.
