Most galaxies are thought to harbor a supermassive black hole at their center, which is basically the universe's way of saying, 'I have a really big appetite.' These enormous objects can weigh millions or even billions of times more than the sun, creating some of the strongest gravitational environments known - strong enough to turn a star into spaghetti, if you're into that kind of metaphor.
When a star passes dangerously close to one of these black holes, destruction isn't always immediate. Some stars survive the encounter and come back for more, generating a fresh burst of light each time. These events, known as repeating partial tidal disruption events (rpTDEs), let astronomers watch the same star get repeatedly nibbled by the same black hole. Wide-field time-domain surveys make this possible by scanning large regions of the sky and tracking objects whose brightness changes - like a cosmic reality show.
Yet some of these systems have been a mystery. Instead of producing similar flares on each return, they become steadily fainter - like a bad sequel that loses steam. For years, theoretical models struggled to explain this behavior.
New research from astrophysicists at Syracuse University suggests a previously underappreciated property of the star might be the key: how fast it was spinning before its first close encounter with the black hole. The study, published in The Astrophysical Journal, was led by doctoral student Ananya Bandopadhyay, along with postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin, all in the Department of Physics, plus collaborators elsewhere.
In a standard tidal disruption event (TDE), the black hole's gravity varies so much across the star that the star is completely torn apart. The debris falls toward the black hole, releasing light over days to months. Black holes don't emit light themselves, but a TDE temporarily supplies material that illuminates the region - a sort of cosmic flashlight.
Not every encounter ends in total destruction. If a star passes close enough to lose only part of its mass, it's a partial TDE. In a repeating partial TDE, the surviving core stays in orbit and returns for more close encounters, shedding more material each time. These passages can happen months or several years apart.
The amount of material stripped depends on the star's internal structure. Bandopadhyay likens a low-mass star to a fluffy meringue - it becomes increasingly susceptible to tidal forces. A higher-mass star is more concentrated toward the center, with an onion-like structure, losing its outer layers while the core stays stable, so mass loss decreases over time.
These structural differences help explain why rpTDEs evolve differently, but one observation has been especially puzzling: of the roughly 10 repeating systems found so far, four show flares that get progressively dimmer. It would seem natural to assume less stripped material means weaker flares, but previous simulations showed the opposite - even with less mass loss, peak brightness stayed about the same.
'We were puzzled by this for two years,' Bandopadhyay says.
Their earlier work revealed another consequence of tidal forces: they apply torque, spinning the star faster after each encounter. That increased rotation changes how quickly stripped material falls back toward the black hole. Even though less material comes back, it does so over a shorter time, keeping the peak fallback rate - and thus the predicted flare brightness - roughly constant.
To reproduce the fading flares, the researchers needed 'a new ingredient': a star already rotating rapidly before its first encounter. Their new simulations suggest such a star can't be spun up much more during later passages. Without a big rotation increase, the fallback time stays steady, so as less material is stripped, the peak fallback rate drops, and the flare gets fainter - matching observations.
But why would a star approaching a supermassive black hole already be spinning so fast? 'It is also extremely difficult to bind a star to a supermassive black hole so tightly that it orbits in a matter of months, and yet they seem to do so in rpTDEs,' Coughlin says.
A process called the Hills mechanism might explain both the rapid rotation and the tight orbit. In this scenario, a binary star system approaches the black hole, which tears it apart, flinging one star away and capturing the other. Stars in a very close binary can be tidally locked, rotating at the same rate they orbit each other. The tighter the binary, the faster the rotation. To end up on the short orbit observed in rpTDEs, the original binary must have been extremely compact, naturally producing a rapidly spinning star.
'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. 'From a theoretical standpoint, this is a major step forward in our understanding of the physics at play in these systems.'
The implications may extend beyond distant repeating flares. Hills capture might also explain some stars orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way. If so, the same mechanism could help astronomers understand the unusual stellar populations around the black hole in what Coughlin calls 'our own cosmological backyard.'
Materials provided by Syracuse University. Note: Content may be edited for style and length.