Most galaxies are believed to contain a supermassive black hole at their core. These massive objects, which can be millions or even billions of times the mass of the Sun, create some of the most extreme gravitational fields in the universe.
When a star ventures too close to such a black hole, it doesn’t always face immediate destruction. Some stars manage to survive these perilous encounters and return for subsequent close approaches, generating new bursts of light with each pass.
These phenomena, known as repeating partial tidal disruption events (rpTDEs), offer astronomers the rare opportunity to study the same star interacting with the same black hole multiple times. Such observations are made possible through wide-field time-domain surveys, which systematically scan vast areas of the sky and monitor objects whose brightness fluctuates over time.
Despite these advances, certain systems have posed puzzling questions for researchers. Instead of emitting flares of consistent brightness upon each return, these systems display progressively dimmer emissions. For years, theoretical models struggled to account for this observed behavior.
A recent study by astrophysicists at Syracuse University proposes that a star’s initial rotation rate might hold the key to solving this mystery. The findings, published in The Astrophysical Journal, were led by doctoral student Ananya Bandopadhyay, alongside postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin from the Department of Physics, as well as collaborators from other institutions.
Understanding How Black Holes Disrupt Stars
During a typical tidal disruption event (TDE), the intense gravitational forces from a black hole vary dramatically across a nearby star, ultimately tearing it apart completely.
The resulting stellar debris spirals toward, or “accretes” onto, the black hole. As this material loses energy, it emits light over durations spanning days to months.
Since black holes themselves do not emit light, a TDE serves as a temporary beacon, illuminating the surrounding region and allowing astronomers to indirectly probe these otherwise invisible entities.
Not all stellar encounters result in total annihilation. If a star approaches a black hole without crossing the threshold for complete disruption, it may shed only a portion of its mass, resulting in a partial TDE. In repeating partial TDEs, the star’s resilient core continues orbiting and makes repeated close approaches, releasing more material each time. These cycles can occur over intervals ranging from months to several years.
The Mystery of Diminishing Flares
The quantity of material stripped from a star during successive encounters depends partly on its internal structure. Bandopadhyay compares a low-mass star to a delicate meringue. Such a star becomes increasingly vulnerable to the black hole’s tidal forces with each pass.
In contrast, a higher-mass star maintains a denser core with an onion-like layered structure. It may lose outer layers while preserving its inner core, leading to a gradual reduction in the amount of material shed during successive encounters.
These structural variations help explain why not all rpTDEs evolve identically. However, one particular observation has proven especially challenging to decipher. Among the approximately ten repeating systems identified so far, four have exhibited flares that grow progressively fainter over time.
At first glance, it might seem logical to assume that smaller quantities of stripped material would simply produce weaker flares. Yet previous hydrodynamical simulations revealed a surprising complication: even when a star lost less material during each passage, the models still predicted flares with roughly equivalent peak brightness.
“We were puzzled by this for two years,” Bandopadhyay reflects.
Earlier research had uncovered another critical factor related to the black hole’s tidal forces. Beyond drawing material away from the star, these forces also exert torque, accelerating the star’s rotation after each close encounter.
This heightened rotation alters the timeline for stripped material to return to the black hole. Although less material comes back, it does so over a shorter period. This more concentrated inflow helps maintain a similar peak fallback rate, thereby sustaining comparable flare brightness levels.
Introducing the Spinning Star Factor
To replicate the fading flares observed by astronomers, the team realized they needed what Bandopadhyay describes as “a new ingredient”: a star that was already rotating rapidly before its initial encounter with the black hole.
The updated simulations indicate that a pre-existing fast-spinning star cannot be spun further to the same degree during subsequent passages. Without significant increases in rotation after each encounter, the time required for stripped material to return to the black hole remains relatively constant.
This shift has profound implications. As progressively less material is stripped from the star, the peak fallback rate also diminishes. Consequently, the predicted flare intensity weakens with each encounter, aligning with the observed dimming trends.
Tracing the Origin of Captured Stars
The discovery raises another compelling question: Why would a star approaching a supermassive black hole already be spinning at such high speeds?
“It is also extremely difficult to ‘bind’ a star to a supermassive black hole so tightly that it orbits the black hole in a matter of months, and yet they seem to do so in rpTDEs,” explains Coughlin.
The Hills mechanism offers a plausible explanation for both the star’s rapid rotation and its unusually tight orbit.
In this scenario, two stars orbiting closely around each other approach a supermassive black hole. The black hole’s gravitational pull tears the binary pair apart. One star is ejected at high speed, while the other becomes trapped in orbit around the black hole.
Stars in extremely close binary systems often become tidally locked, meaning they rotate at the same rate as their orbital period around each other. The tighter the binary, the shorter the orbital period—and the faster a tidally locked star must spin.
To place a captured star on the short orbit characteristic of rpTDEs, the original binary system would need to be extraordinarily compact. This same configuration would naturally produce a rapidly spinning, tidally locked star prior to its capture by the black hole.
“Ananya’s work demonstrates that each of these peculiarities can be explained by the same underlying phenomenon: the tidal destruction of a binary system and the capture of one of the stars,” Coughlin notes. “From a theoretical standpoint, this represents a significant advancement in our understanding of the physics governing these systems.”
Implications for Our Galactic Neighborhood
The significance of this research extends beyond distant repeating flare systems. Coughlin points out that the Hills mechanism may also play a role in shaping the stellar population surrounding Sagittarius A*, the supermassive black hole at the center of the Milky Way.
If confirmed, the same process responsible for fading rpTDE flares could offer valuable insights into some of the unusual stars orbiting our galaxy’s central black hole—what Coughlin refers to as “our own cosmological backyard.”
