Stellar spin may explain why repeated black hole flares grow dimmer

Supermassive black holes at the cores of galaxies possess immense gravity and can pull stars in dangerously close. When stars venture too near, they are not entirely destroyed but instead experience repeated partial tidal disruption events (rpTDEs). Each encounter results in a flare of light as the star's material falls toward the black hole. However, in some cases, researchers have observed that these successive flares grow progressively dimmer. Theoretical models have struggled to explain this phenomenon.

A team of astrophysicists from Syracuse University, led by doctoral student Ananya Bandopadhyay, postdoctoral researcher Benjamin Amend, and associate professor Eric Coughlin, have discovered that the star's spin may hold the key to understanding the dimming of rpTDE flares. In a typical tidal disruption event, a black hole's gravitational pull tears a star apart, causing the disrupted stellar debris to fall inward and emit light as it loses energy.

In a partial TDE, the surviving stellar core continues to orbit the black hole, gradually losing material with each close pass. The amount of mass lost depends on the star's internal structure. For low-mass stars, which are more like fluffy meringues, the tidal forces can strip away more of their outer layers, while higher-mass stars, with a more centrally concentrated structure, lose a smaller fraction of their mass over time.

The researchers found that the dimming pattern in a subset of these repeating systems could be attributed to the star's initial spin. A star already spinning rapidly before encountering the black hole does not experience significant spin-up during each passage. This prevents the star from losing material at a consistent rate, causing the peak fallback rate and the predicted brightness of the flare to decline over time.

The authors propose that the Hills mechanism may be responsible for the rapid initial spins observed in these rpTDEs. In this scenario, a pair of closely orbiting stars passes near a supermassive black hole, tearing the binary apart and ejecting one star while capturing the other. If the binary system is extremely tight, the captured star becomes tidally locked and spins rapidly before its capture.

The findings of this study provide a significant step forward in understanding the complex physics at play in these rpTDE systems. The rapid initial stellar rotation could also help explain the properties of some stars orbiting the supermassive black hole at the center of the Milky Way, Sagittarius A*.