Stellar spin may explain why repeated black hole flares grow dimmer

Astronomers have uncovered a potential explanation for why repeated flares from stars interacting with supermassive black holes gradually grow dimmer over time. According to a recent study published in The Astrophysical Journal, the key factor appears to be the rotational speed of the star itself. This discovery comes after years of uncertainty regarding the observed behavior of recurring partial tidal disruption events (rpTDEs). Supermassive black holes, located at the centers of most galaxies, possess masses ranging from millions to billions of times that of the Sun. When a star ventures too close to such a black hole, the immense gravitational forces can either fully destroy the star or partially tear it apart, depending on how close the approach occurs. If the destruction is incomplete, the remaining portion of the star continues to orbit the black hole, periodically losing more material in subsequent encounters. These interactions produce observable flares of light, offering scientists a rare chance to observe the same cosmic event multiple times. Researchers have long noted that some of these flares appear to decrease in brightness with each recurrence. However, existing models struggled to account for this phenomenon. A team of astrophysicists based at Syracuse University, including doctoral student Ananya Bandopadhyay, postdoctoral researcher Benjamin Amend, and associate professor Eric Coughlin, sought to address this discrepancy through detailed simulations and analysis. The team’s findings suggest that the rate at which a star loses mass during these encounters is influenced significantly by its internal structure and initial spin. Low-mass stars, likened to a fluffy meringue, tend to lose more material quickly due to their loosely packed composition. High-mass stars, on the other hand, resemble an onion with a dense core, shedding only the outer layers initially and preserving the core for longer periods. This structural difference helps explain variations in the brightness of flares, yet it did not fully resolve the issue of progressive dimming. Earlier simulations indicated that even as less material fell into the black hole with each encounter, the resulting flares remained nearly constant in brightness. This contradiction prompted further investigation. Bandopadhyay and her colleagues discovered that the black hole’s tidal forces not only strip away material from the star but also apply a torque that increases the star’s rotational speed with each close approach. While this increased spin caused the material to fall back toward the black hole more quickly, it also meant that the overall amount of material available for accretion diminished over time. To align their simulations with actual observations showing a decline in flare brightness, the researchers introduced a crucial variable: the star’s initial spin rate. They found that if a star was already rotating rapidly before encountering the black hole, the combination of its pre-existing angular momentum and the additional spin induced by the black hole’s tidal forces could lead to a noticeable decrease in the brightness of subsequent flares. This insight provides a plausible mechanism for the observed dimming patterns in four known rpTDE systems. It also highlights the importance of considering a star’s intrinsic properties when modeling these complex interactions. The research builds upon earlier studies that explored the dynamics of tidal disruption events. By incorporating factors such as stellar spin and internal structure, the team has offered a more comprehensive framework for understanding how these cosmic phenomena evolve over time. The implications of this work extend beyond just explaining the dimming of flares. It opens new avenues for studying the physics of both stars and black holes, particularly in environments where repeated interactions occur. Future observations will aim to confirm whether the initial spin of stars plays a role in other similar events, potentially refining current models of stellar evolution near massive black holes. Scientists continue to monitor the skies using wide-field time-domain surveys, searching for changes in brightness that indicate ongoing interactions between stars and black holes. With improved models and observational techniques, researchers hope to gain deeper insights into the intricate dance between celestial bodies and the powerful forces that govern them.