Astronomers may have found an explanation for a puzzling pattern produced when a star repeatedly survives close encounters with a supermassive black hole. New hydrodynamical simulations indicate that a star already rotating rapidly before its first passage can shed progressively less material and generate a dimmer flare on every return, researchers reported in a study published in The Astrophysical Journal.
The events are called repeating partial tidal disruption events. A black hole's uneven gravitational pull strips part of a passing star, but the stellar core survives in orbit and returns months or years later. Material falling back toward the black hole releases energy as light, allowing astronomers to examine an otherwise invisible object indirectly. Roughly 10 repeating systems have been identified, according to Syracuse University, and four display steadily fading flares.
Earlier simulations presented a problem. They showed that a star losing less mass on successive passages should also spin faster because the black hole applies torque. The smaller amount of debris then returns over a shorter interval, keeping the peak fallback rate and predicted brightness nearly constant. That result did not match the declining light seen in several real systems.
A team led by Syracuse doctoral researcher Ananya Bandopadhyay added rapid initial stellar rotation to its models. In that case, later passages produce much less additional spin-up, so the debris return time remains comparatively stable. As each encounter strips less mass, the peak fallback rate falls and the simulated flare fades. The result offers a possible physical explanation, although observations have not yet directly established the proposed initial spin.
The researchers said the Hills mechanism could explain both the rotation and the unusually tight orbit. Under that scenario, a closely orbiting pair of stars approaches a supermassive black hole; gravity ejects one member and captures the other. Stars in a compact binary can be tidally locked and rotate quickly before the separation, naturally providing the condition required by the simulations.
The model may also inform research on stars near Sagittarius A*, the supermassive black hole at the Milky Way's center, because a similar capture process may have placed some of them there. Future wide-field surveys can discover more repeating events and measure how their flares change, giving researchers a larger sample with which to test whether rapid initial spin consistently accounts for the fading pattern.
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