At the hearts of galaxies, supermassive black holes occasionally seize passing stars and hold them in a recurring gravitational embrace — each close approach stripping away a little more of the star's outer layers and producing a burst of light. For two years, researchers at Syracuse University puzzled over why some of these repeating flares grew progressively dimmer, a pattern their models could not explain. The answer, they found, lay not in the encounters themselves but in the star's hidden history: a rapid spin inherited from a tight binary partnership long before the black hole ever claim
Star's Pre-Existing Spin Explains Dimming Pattern in Black Hole Flares
The star's spin is a fossil record of its past
Mark
Why does it matter that we understand why these flares are getting dimmer?
Mimi
Because it tells us something fundamental about how stars end up in these extreme orbits in the first place. The dimming pattern is like a fingerprint—it points back to a specific origin story.
Mark
And that origin story is the Hills mechanism?
Mimi
Yes. A binary pair of stars gets torn apart by a black hole, and one gets captured. The star that gets captured is already spinning fast because of how tightly it was bound to its partner.
Mark
So the star's spin is a kind of memory of its past?
Mimi
Exactly. It's a fossil record written into the star's rotation. Once you know to look for it, you can read the history.
Mark
Does this help us understand stars around Sagittarius A*?
Mimi
Potentially, yes. Some of those stars might have arrived the same way. If we can identify which ones were captured through Hills disruption, we learn something about the dynamical history of our galaxy's center.
Mark
What was the hardest part of solving this puzzle?
Mimi
Recognizing that the star's initial spin mattered at all. The models were sophisticated, but they were missing something simple—a property the star brought with it from before.
Le Pouls
- Four known systems defied all predictions — their repeated stellar flares dimmed with each pass, even as existing models insisted brightness should hold steady.
- Two years of failed simulations created mounting pressure on theorists, who knew something fundamental was absent from their picture of these cosmic encounters.
- The breakthrough came when researchers recognized that a star's pre-existing rapid spin — not the black hole's torque — was the missing variable controlling how stripped material falls back and glows.
- That spin traces to a violent origin: a binary star pair torn apart by the Hills mechanism, leaving one star captured and already whirling fast before its first close encounter.
- The single mechanism now resolves multiple mysteries at once — orbital speed, stellar spin, and dimming flares — and points toward new explanations for the unusual stars circling our own galaxy's central black hole, Sagittarius A*.
At the hearts of galaxies, supermassive black holes occasionally seize passing stars and hold them in a recurring gravitational embrace — each close approach stripping away a little more of the star's outer layers and producing a burst of light. For two years, researchers at Syracuse University puzzled over why some of these repeating flares grew progressively dimmer, a pattern their models could not explain. The answer, they found, lay not in the encounters themselves but in the star's hidden history: a rapid spin inherited from a tight binary partnership long before the black hole ever claimed it. In solving this puzzle, they may have illuminated the deeper story of how the violent centers of galaxies are shaped.
Most galaxies conceal a supermassive black hole at their center, and occasionally a star wanders close enough to be partially torn apart. Sometimes the star survives, loses some of its outer material, and returns months or years later for another close pass — producing another flare of light. Astronomers call these repeating partial tidal disruption events, and about a dozen have been identified. Most behave as expected. But four of them showed something strange: each successive flare was dimmer than the last.
For two years, researchers at Syracuse University could not explain this. The intuitive answer — that the star was losing less material with each encounter — didn't hold up. Their simulations showed that even with decreasing mass loss, the brightness of the flare should have remained roughly constant. Something was missing.
The missing piece was the star's spin. Doctoral student Ananya Bandopadhyay and her colleagues realized that if a star arrived at its first encounter already spinning rapidly, the additional spin-up imparted by the black hole's tidal torque would be negligible. Without that extra rotational boost, the timescale of material falling back toward the black hole stays fixed — and so, as less material is stripped with each pass, the peak brightness finally drops, matching what astronomers observed.
This raised a further question: why would the star be spinning so fast in the first place? The answer lies in the Hills mechanism. When a tight binary pair of stars ventures near a supermassive black hole, the black hole can tear them apart — ejecting one and capturing the other. In a close binary, both stars are tidally locked, spinning as fast as they orbit each other. The tighter the binary, the faster the spin. A binary compact enough to place its captured star on the short orbit seen in these systems would leave that star already whirling rapidly upon capture.
This single origin story — the tidal destruction of a binary — simultaneously explains why rpTDE stars orbit their black holes so quickly, why they spin so fast, and why some of their flares dim over time. The findings, published in The Astrophysical Journal, may also illuminate the properties of stars orbiting Sagittarius A*, our own galaxy's central black hole, some of which may have arrived there through the same Hills capture process.
Most galaxies harbor a supermassive black hole at their center—a gravitational monster with the mass of millions or billions of suns. When an unlucky star wanders too close, the black hole's tidal forces can tear it apart. But sometimes a star survives these encounters, barely. It loses some of its outer material and keeps orbiting, only to return for another close pass months or years later, producing another burst of light. Astronomers call these repeating partial tidal disruption events, or rpTDEs, and they offer a rare chance to watch the same cosmic drama unfold multiple times.
About a dozen of these systems have been identified so far. Most behave as theory predicts. But four of them showed something strange: with each successive pass, the flares grew dimmer. For two years, researchers at Syracuse University couldn't figure out why. The obvious explanation—that the star was losing less material with each encounter—didn't work. Their computer simulations showed that even as the amount of stripped material decreased, the brightness of the resulting flare should have stayed roughly the same. Something was missing from the model.
The answer, it turned out, was hiding in plain sight: the star's spin. Doctoral student Ananya Bandopadhyay and her colleagues realized that the black hole's tidal forces do more than just pull material away from the star. They also exert a torque that spins the star faster with each close approach. If a star arrived at its first encounter already spinning rapidly, this additional spin-up would be minimal. Without that extra rotational boost, the timescale over which stripped material falls back toward the black hole would remain constant. As the star shed less material with each pass, the peak rate of that fallback—and therefore the brightness of the flare—would finally decline, matching what astronomers were actually seeing.
But this raised a new question: why would a star be spinning so fast to begin with? The answer traces back to the star's past. The most likely scenario involves what astrophysicists call the Hills mechanism. Imagine two stars orbiting each other in a tight binary pair. If this pair ventures near a supermassive black hole, the black hole's gravity can tear them apart, ejecting one star and capturing the other. In a close binary, the two stars become tidally locked—each one rotates on its axis at the same rate the pair orbits each other. The tighter the binary, the faster that orbital period, and therefore the faster each star spins. A binary tight enough to leave its captured star on the short orbit observed in rpTDEs would have to be extraordinarily compact, which means the captured star would already be spinning rapidly when the black hole seized it.
This single mechanism—the tidal destruction of a binary system—explains multiple puzzles at once. It accounts for why rpTDE stars orbit their black holes so quickly, why they spin so fast, and why the flares from some of these systems grow progressively dimmer. "From a theoretical standpoint, this is a major step forward in our understanding of the physics at play in these systems," said Eric Coughlin, the associate professor who supervised the work. The findings, published in The Astrophysical Journal, may also shed light on the stars orbiting Sagittarius A*, the supermassive black hole at the center of our own galaxy. Some of those stars may have arrived at their current orbits through the same Hills capture mechanism, which could help explain their observed properties and offer a window into the violent dynamics that shape the centers of galaxies.
Citations marquantes
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
We were puzzled by this for two years.— Ananya Bandopadhyay, doctoral student
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