A team led by Syracuse University doctoral student Ananya Bandopadhyay has published a study in *The Astrophysical Journal* (August 2026) that resolves a two-year-old puzzle in the study of repeating partial tidal disruption events (rpTDEs): why do 4 of the roughly 10 known systems grow dimmer with each successive flare?
Background: stars that survive being shredded
In a classical tidal disruption event (TDE), a star passing near a supermassive black hole is torn apart in a single, usually fatal, encounter. In rpTDEs, however, the star survives—only its outer layers are stripped—and it returns on its orbit months or years later, producing a new flare each pass. About 10 such systems are known, and in 4 of them each outburst is fainter than the last, which simple mass-loss arguments could not explain.
Key finding: pre-capture spin rate
The Syracuse team's hydrodynamic simulations show the decisive variable is the star's spin rate before being captured by the black hole:
- Tidal torques normally spin the star up after each passage, shortening debris fallback times and sustaining flare brightness.
- A star already spinning rapidly at first encounter gains little additional angular momentum, so fallback times no longer shorten; as mass decreases, peak fallback rates drop and flares fade.
- Low-mass "fluffy" stars can brighten as more mass is stripped per pass, while high-mass "onion-like" stars fade only if they were fast rotators before capture.
- Theory: rpTDEs are no longer isolated oddities but a unified phenomenon, comparable using host-binary tightness and stellar internal structure.
- Observations: rpTDE rates can probe the density of compact binaries around supermassive black holes, constrain nuclear stellar dynamics, and offer independent black-hole mass estimates; the framework may also explain some hypervelocity stars near Sgr A*.
- Ananya Bandopadhyay et al., "Spin-up at First Encounter Drives Fading in Repeating Partial Tidal Disruption Events," *The Astrophysical Journal*
- ScienceDaily: sciencedaily.com/releases/2026/08/260822015119.htm
- Syracuse University team: Ananya Bandopadhyay, Benjamin Amend, Eric Coughlin
- Hills capture mechanism: Hills 1988 and modern binary-dynamics reviews
Origin of the fast spin: the Hills mechanism
The paper explains the fast pre-capture spin via Hills capture: a tight binary passes the black hole and is tidally split—one star is ejected as a hypervelocity star, the other is captured on a short-period orbit. In such tight binaries the stars are tidally locked, so their spin equals the (short) orbital period. Since all observed rpTDEs have short orbital periods (months to years), their progenitor binaries must have been very tight, making fast-rotating captured stars the natural norm.
This unifies the observations:
1. 4/10 systems with fading flares — Hills-capture products, fast rotators before being bound. 2. 6/10 systems with steady flares — single stars directly captured, still being spun up by tidal torques. 3. Short orbital periods across the sample — a signature of tight-binary Hills capture.
Broader significance
The team plans to test the theory with a larger sample—the Vera Rubin Observatory's LSST survey, beginning in 2025, is expected to discover hundreds of new rpTDE candidates.