A new study published in The Astrophysical Journal argues that the spin of a star before its first close encounter with a supermassive black hole could explain one of the more persistent puzzles in repeating partial tidal disruption events: why some stellar flares grow systematically fainter with every pass, rather than staying roughly the same brightness.
The research comes from a team at Syracuse University, led by doctoral student Ananya Bandopadhyay, working alongside postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin, all in the Department of Physics, with collaborators at other institutions.
What repeating partial tidal disruption events actually are
Most galaxies are thought to harbour a supermassive black hole at their centre, objects that can weigh millions or even billions of times more than the sun. When a star passes close enough, the black hole’s gravity can differ so strongly from one side of the star to the other that material is stripped away. If the star survives, its core remains in orbit and returns for further close encounters, shedding more mass each time. These are repeating partial tidal disruption events, and wide-field time-domain sky surveys have made it possible to observe them repeatedly in the same systems.
Of the roughly ten such repeating systems identified so far, four have shown something that did not fit the existing models: their flares become progressively dimmer. It might seem intuitive to assume that less stripped material produces a weaker flare. Previous hydrodynamical simulations, however, suggested otherwise. Even when the star lost less mass on each pass, the models still predicted flares of roughly the same peak brightness. ‘We were puzzled by this for two years,’ Bandopadhyay says.
The reason, the earlier simulations showed, lies in what the black hole’s tidal forces do beyond stripping mass: they also apply torque, spinning the star faster after each close encounter. A faster-spinning star causes the stripped material to return toward the black hole more quickly. Even though less material arrives, it does so over a shorter window, keeping the peak fallback rate, and therefore the predicted flare brightness, roughly constant.
Pre-existing spin: the ingredient the models were missing
To reproduce the fading flares that astronomers actually observe in repeating partial tidal disruption events, the team introduced what Bandopadhyay called ‘a new ingredient’: a star that was already rotating rapidly before its first encounter. The simulations suggest that a star arriving with high spin cannot be spun up nearly as much by subsequent passes. Without that additional torque-driven acceleration, the time for stripped material to fall back stays relatively constant. As less material is stripped with each encounter, the peak fallback rate falls accordingly, and the predicted flare dims. That matches what has been seen.
The finding immediately raises a second question: how would a star arrive at a supermassive black hole already spinning quickly, and in an unusually tight orbit? Both observations are difficult to explain in isolation. Coughlin notes that ‘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.’
The team’s proposed answer is the Hills mechanism. In this scenario, two stars in a close binary system approach a supermassive black hole together. The black hole’s gravity tears the pair apart: one star is flung away, while the other is captured into orbit. Stars in a very tight binary can become tidally locked, rotating on their own axes at the same rate as the pair’s orbital period. The more compact the binary, the faster that rotation must be. A sufficiently compact binary would naturally produce a captured star that is already spinning rapidly, and whose orbit around the black hole is correspondingly short. ‘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 says.
The study also draws a possible connection to Sagittarius A*, the supermassive black hole at the centre of the Milky Way. Coughlin suggests Hills capture may account for some of the unusual stellar populations orbiting it, which he describes as sitting in ‘our own cosmological backyard.’
Funding behind the research
The work was supported by NASA through two separate programmes. Bandopadhyay received support through the FINESST programme, grant 80NSSC24K1548, and through the Syracuse University Research Excellence Dissertation Grant. Amend and Coughlin received support through NASA’s Astrophysics Theory Programme, grant 80NSSC24K0897, and through Chandra Award Number 25700383. The paper is published as Bandopadhyay et al. in The Astrophysical Journal, volume 1007, number 2, DOI 10.3847/1538-4357/ae8f31.
