Stellar spin may explain why repeated black hole flares grow dimmer

A hydrodynamical simulation of a star being ripped apart by the tidal forces of a supermassive black hole. Credit: NASA/ S. Gezari (JHU)/ J. Guillochon (UCSC)

The study found that the dimming of repeated black hole flares may be explained by the star's initial rotation. Researchers observed that a star already spinning rapidly before its first encounter with the black hole prevents it from being significantly spun up during each passage. Without that additional spin-up, the timescale over which the stripped material falls back remains relatively constant. As the star loses less material with each encounter, the peak fallback rate—and the predicted brightness of the flare—can finally decline.

The study was led by doctoral student Ananya Bandopadhyay, working with postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin—all in the Department of Physics—as well as colleagues at other institutions. The findings suggest that the dimming of repeated black hole flares may be due to the star's initial rotation, which affects how much material is lost and how quickly it falls back.

The study's results could help explain the properties of stars in what Coughlin calls "our own cosmological backyard." The findings provide a theoretical explanation for the observed dimming pattern in four of the ten identified repeating partial tidal disruption events. The study highlights the importance of considering a star's initial rotation when analyzing these phenomena.

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