Across the observable universe, certain stars have been caught in a slow gravitational waltz with supermassive black holes — returning again and again, each time a little dimmer than before. For years, this fading defied explanation, as existing models predicted the brightness should hold steady. Now, a team from Syracuse University has traced the mystery to something as intimate as a star's own spin, revealing how the deep past of a stellar pair can quietly shape the light we see millions of years later.
Rapidly spinning stars explain why black hole flares fade with each encounter
A star can brush past a supermassive black hole, lose a piece of itself, and survive.
Mark
Why does it matter that we understand why these flares fade? Aren't we already detecting the events?
Mimi
Detection is one thing. Understanding is another. Right now, when we see a star flaring repeatedly near a black hole, we're watching a cosmic laboratory. Each flare tells us something about the star and how it got there. If we can decode what the fading pattern means, we can work backward to figure out what the star was like before it was captured.
Mark
So the spin of the star is like a fingerprint of its past?
Mimi
Exactly. If a star is spinning rapidly and its flares are dimming in a particular way, that tells us it probably came from a tight binary that was torn apart by the black hole. We're reading the violence of its capture in the light it sends us now.
Mark
The Hills mechanism—is that something that happens often, or is it rare?
Mimi
It's theoretically possible whenever a binary system passes close enough to a supermassive black hole. We know it happens because we see the signatures. But whether it's common or rare, we're still figuring out. What's remarkable is that one mechanism explains so many things at once: the tight orbits, the fast spins, the dimming flares.
Mark
What about AT2018fyk, the one that doesn't fit the pattern?
Mimi
That's the interesting outlier. Its flares dropped by a factor of ten, much steeper than the model predicts. The black hole there is extremely massive, which means the star gets pulled into a regime where Einstein's relativity becomes crucial. There might be chaotic effects we're not fully accounting for yet. It's a reminder that the universe is more complicated than any single model.
Mark
Does this help us understand what's happening at Sagittarius A*, our own black hole?
Mimi
Potentially. The same capture mechanism could be at work there too. If we ever detect repeating partial disruptions around Sagittarius A*, this framework would help us interpret them. We'd be reading the history of stars that survived encounters with the black hole at the center of our galaxy.
Der Puls
- Repeated flares from stars grazing supermassive black holes have been growing mysteriously dimmer with each pass, contradicting every model astronomers trusted.
- The tension deepened because simulations kept producing steady brightness — the dimming simply refused to appear, suggesting something fundamental was missing from the physics.
- Doctoral student Ananya Bandopadhyay's team introduced one changed condition — a star already spinning rapidly before its first encounter — and the dimming finally emerged in the data.
- Rapid pre-existing spin means the black hole can transfer little additional rotation, so each successive pass strips less material and produces a fainter flare, matching real observations.
- The Hills mechanism — a black hole tearing apart a binary star system — may naturally supply both the tight orbits and the fast spin needed, unifying three separate puzzles into one origin story.
- The framework now lets astronomers read fading flare sequences like a biographical record, reconstructing a star's properties and the violent capture event that set its fate.
Across the observable universe, certain stars have been caught in a slow gravitational waltz with supermassive black holes — returning again and again, each time a little dimmer than before. For years, this fading defied explanation, as existing models predicted the brightness should hold steady. Now, a team from Syracuse University has traced the mystery to something as intimate as a star's own spin, revealing how the deep past of a stellar pair can quietly shape the light we see millions of years later.
A star can survive a brush with a supermassive black hole, losing part of itself before escaping — only to return months or years later for another encounter. In several known systems, this cycle repeats, but each returning flare is dimmer than the last. That progressive fading has puzzled astronomers for years, because their best simulations consistently predicted the brightness should remain roughly constant.
A team led by Syracuse University doctoral student Ananya Bandopadhyay found the missing piece in the star's own rotation. When tidal forces strip a star's outer layers, the infalling material produces a bright flare — but the black hole also exerts torque on the surviving star, altering how fast it spins. In earlier models, slowly rotating stars gained significant spin with each encounter, making them progressively easier to strip and keeping flare brightness steady. The dimming never appeared.
Bandopadhyay's team changed one starting condition: they modeled stars already spinning rapidly before the first encounter. When a star's rotation matched the orbital frequency at closest approach, the black hole transferred far less additional spin — sometimes even slowing the star slightly. With rotation remaining stable across passages, the increasingly dense surviving core shed less material each time, and the peak flare brightness declined — precisely as observed in systems like ASASSN-14ko, AT2018fyk, and AT2022dbl.
In simulations of a Sun-mass star, the peak fallback rate dropped by a factor of 1.5 between the first and second encounters, then continued fading gradually. The pattern held across stars of different masses and evolutionary stages. One outlier, AT2018fyk, showed a steeper decline of roughly tenfold, hinting at additional relativistic effects tied to its exceptionally massive black hole.
The deeper question — why these stars would already be spinning so fast — finds a natural answer in the Hills mechanism. When a tightly orbiting binary star system passes near a supermassive black hole, the black hole can tear the pair apart, flinging one star outward and locking the other into a close orbit. Because tight binaries are often tidally locked, their stars spin at the same rate the pair orbits each other — and the tighter the binary, the faster that spin. The same violent event that captures a star near a black hole leaves it rotating rapidly.
This transforms three separate puzzles — tight stellar orbits, rapid rotation, and fading flares — into a single coherent origin story. Published in The Astrophysical Journal, the work opens a new way to read repeating flares as biographical records, allowing astronomers to reconstruct a star's properties before capture and trace the cataclysm that first set it on its present path.
A star can brush past a supermassive black hole, lose a piece of itself, and survive. Months or years later, it returns for another encounter. In several known systems scattered across the sky—ASASSN-14ko, AT2018fyk, AT2022dbl, and others—this cycle repeats. But with each return, the flare grows dimmer. That dimming has puzzled astronomers for years, defying the predictions of their best models.
Now a team led by Syracuse University doctoral student Ananya Bandopadhyay has found an answer hiding in plain sight: the star's spin. Using hydrodynamical simulations, the researchers discovered that stars already rotating rapidly before their first encounter with a black hole behave differently than slowly spinning ones. When a star is already spinning fast, the black hole's gravity cannot easily add more rotation during each close pass. That stability in spin, in turn, explains why less material gets stripped away with each successive encounter—and why the resulting flares fade.
The mechanism works like this. When a black hole pulls a star close enough, tidal forces—the difference in gravitational pull across the star's surface—can tear away its outer layers without destroying the core entirely. As that stripped material falls toward the black hole, it heats up and radiates energy, producing a bright flare. But the black hole does more than just remove material. Its gravity also exerts torque on the star, changing how fast it spins. In earlier simulations, stars that began rotating slowly could gain significant additional spin during repeated encounters. That extra rotation made the star easier to strip in subsequent passages, keeping the fallback rate—and thus the brightness of the flare—roughly constant. The dimming never materialized.
Bandopadhyay's team changed one crucial starting condition. They modeled stars that were already spinning rapidly before their first encounter. When the star's rotation matched the orbital frequency at its closest approach to the black hole, something different happened. The black hole transferred far less additional rotation. In some cases, the encounter even slowed the star slightly. With the spin remaining relatively stable from passage to passage, the surviving star—now denser from losing its outer layers—shed progressively less material. The peak fallback rate declined with each flare, matching what astronomers actually observe.
In one simulation of a Sun-mass star rotating in the same direction as its orbit, the peak fallback rate dropped by a factor of 1.5 between the first and second encounters, then continued declining more gradually. Similar fading patterns emerged in simulations of three-solar-mass stars at different evolutionary stages. The model proved consistent with observations of systems like eRASSt-J045650 and AT2021aeuk, where flare peaks have decreased by factors of two or less. One system, AT2018fyk, showed a much steeper decline—roughly a factor of ten—suggesting additional physics at work, possibly relativistic effects given the extreme mass of its black hole.
But rapid stellar rotation raises its own question: Why would a star orbiting a supermassive black hole already be spinning so fast? The answer may lie in the Hills mechanism, a scenario that explains two seemingly separate puzzles at once. Imagine two stars in a tight binary system—orbiting each other closely and rapidly. When this pair passes near a supermassive black hole, the black hole's gravity can tear the binary apart, flinging one star away and trapping the other on a tight orbit around the black hole. Very close binaries are often tidally locked, meaning each star rotates at the same rate the pair orbits each other. Tighter binaries orbit faster, so their stars spin faster too. The same violent disruption that places a star on a short-period orbit around a black hole naturally leaves it spinning rapidly.
This connection transforms the puzzle into a coherent picture. The tight orbits that allow stars to return to the black hole every few months, the rapid rotation that explains the dimming flares, and the mechanism that captures stars in the first place—all three emerge from a single phenomenon: the tidal destruction of a binary system. "From a theoretical standpoint, this is a major step forward in our understanding of the physics at play in these systems," said Eric Coughlin of Syracuse's Department of Physics. The work, published in The Astrophysical Journal, opens a new way to read the history written in these repeating flares. Astronomers studying the same star interacting with the same black hole multiple times can now use the changing brightness of the flares to reconstruct the star's properties before capture—and to trace the violent events that put it on its present orbit.
Bemerkenswerte Zitate
We were puzzled by this for two years— Ananya Bandopadhyay, Syracuse University doctoral student
From a theoretical standpoint, this is a major step forward in our understanding of the physics at play in these systems— Eric Coughlin, Syracuse University Department of Physics
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