Stars That Survive Repeated Black Hole Encounters Produce Progressively Fainter Flares, Study Finds

Astrophysicists at Syracuse University have proposed an explanation for a puzzling pattern in stars that survive repeated close encounters with a supermassive black hole: instead of producing similarly bright flares each time, some grow progressively fainter. The study, published in The Astrophysical Journal, was led by doctoral student Ananya Bandopadhyay, working with postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin in the university's Department of Physics, along with collaborators at other institutions.

In a standard tidal disruption event (TDE), a black hole's gravity varies so strongly across a passing star that the star is completely torn apart, with the resulting debris falling toward the black hole and releasing light over days to months. In a partial TDE, a star that does not cross the threshold for total disruption loses only part of its mass; its surviving core stays in orbit and returns for additional close passes, months or years apart, shedding more material each time. These are called repeating partial tidal disruption events, or rpTDEs.

Of the roughly 10 repeating systems identified so far, four have shown flares that become progressively dimmer. Previous hydrodynamical simulations struggled to explain this: even when a star lost less material during each pass, the black hole's tidal forces also spin the star up further after every encounter, and the more concentrated returning material was predicted to keep producing flares of about the same peak brightness. 'We were puzzled by this for two years,' Bandopadhyay said.

The new study adds a missing ingredient: a star that was already spinning rapidly before its very first encounter with the black hole. Because such a star cannot be spun up as much during later passages, the time for stripped material to fall back toward the black hole stays relatively steady, so as progressively less material is stripped, the peak fallback rate, and the flare's peak brightness, decreases with each pass, matching what astronomers have observed.

The finding raises the question of why an approaching star would already be spinning so fast, and how it could be bound so tightly to a supermassive black hole that it orbits in a matter of months. The researchers point to the Hills mechanism as a possible explanation: two stars in a tight binary approach a supermassive black hole, whose gravity tears the pair apart, flinging one star away while capturing the other into a tight orbit. Stars in a very close binary can become tidally locked, rotating on their axis at the same rate the pair orbits each other.