According to research published in The Astrophysical Journal by astrophysicists at Syracuse University, repeating partial tidal disruption events—where supermassive black holes repeatedly tear apart surviving stars—produce steadily fading flares because the destroyed stars were already rotating rapidly prior to their first encounter. Led by doctoral student Ananya Bandopadhyay alongside postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin, the study resolves a two-year hydrodynamical modeling puzzle regarding why certain black hole flares dim over successive orbits rather than maintaining constant peak brightness.
How Supermassive Black Holes Shred Stars in Tidal Disruption Events
Most galaxies harbor a supermassive black hole at their center, weighing millions or billions of times more than the sun and generating extreme gravitational environments. In a standard tidal disruption event, according to the research team, a passing star experiences gravitational pulls so intensely varied from one side to the other that the star is completely ripped apart. The resulting stellar debris accretes onto the black hole, releasing light over days or months that allows astronomers to investigate otherwise invisible regions.
However, destruction is not always immediate. If a star passes close to a supermassive black hole without crossing the total disruption threshold, it sheds only part of its mass. During these repeating partial tidal disruption events, or rpTDEs tracked by wide-field time-domain surveys, the surviving stellar core remains in orbit and returns months or years later for additional close encounters.
Did you know? Astronomers have identified roughly 10 repeating partial tidal disruption systems so far, but four of those systems have displayed flares that become progressively dimmer with each return pass.
Why Some Black Hole Flares Keep Fading Over Successive Orbits
The amount of material stripped from a star depends heavily on its internal structure, according to Bandopadhyay, who likens low-mass stars to a “fluffy meringue” that grows increasingly susceptible to tidal forces. Higher-mass stars feature an onion-like internal structure with concentrated central material, allowing them to lose outer layers while their dense cores remain largely intact over successive encounters.
Despite these structural differences, past hydrodynamical simulations presented a major contradiction. Even when models showed a star losing less material during each passage, they still predicted flares with roughly the same peak brightness. Bandopadhyay notes that tidal forces apply torque in addition to stripping material, causing the star to spin faster after each close encounter and quickening the return of debris to maintain a steady peak fallback rate.
The Role of Rapid Stellar Rotation in Fading Flares
To match observational data of fading flares, the Syracuse University team integrated a crucial new property into their simulations: a star that was already spinning rapidly before its initial encounter with the supermassive black hole. According to the study, such a pre-spinning star cannot be spun up nearly as much during subsequent passages.
Without a dramatic increase in rotation speed after each encounter, the time required for stripped material to fall back toward the black hole stays relatively steady. As progressively less material gets stripped from the star during later orbits, the peak fallback rate drops accordingly, producing the progressively fainter flares observed by astronomers.
How the Hills Mechanism Captures and Spins Up Stars
The findings point to a distinct origin for these stars, addressing how a star can become bound so tightly to a supermassive black hole with an orbital period of only a few months. Coughlin points to the Hills mechanism as a likely explanation for both the rapid rotation and the tight orbit.
In this capture scenario, a binary system consisting of two closely orbiting stars approaches a supermassive black hole, which tears the pair apart. One star is hurled away into space, while the other is captured into a tight orbit around the black hole. Stars in very compact binaries become tidally locked, forcing each star to rotate on its axis at the same rate the pair orbits one another. That configuration naturally produces a rapidly spinning star before the black hole captures it.
Connections to the Milky Way Center
The theoretical breakthrough may extend far beyond distant extragalactic systems. Coughlin notes that Hills mechanism captures could also account for some of the unusual stellar populations currently orbiting Sagittarius A*, the supermassive black hole residing at the center of the Milky Way.
By applying the same tidal destruction and binary capture physics studied in repeating partial tidal disruption events, astrophysicists gain a clearer framework for understanding stellar dynamics in our own cosmological backyard.
Frequently Asked Questions
What is a repeating partial tidal disruption event?
A repeating partial tidal disruption event occurs when a supermassive black hole strips away part of a passing star’s mass during a close encounter, leaving a surviving core that orbits back for repeated passes over months or years.
Why do some black hole flares become fainter over time?
According to Syracuse University research, if a star is already rotating rapidly before its first encounter—likely due to being ripped from a tight binary system via the Hills mechanism—subsequent passages do not significantly increase its spin, causing the fallback rate of stripped material and the resulting flare brightness to decrease.
What is the Hills mechanism?
The Hills mechanism is a gravitational process where a binary star system passes close to a supermassive black hole, resulting in one star being ejected from the system while the companion star is captured into a close orbit around the black hole.
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