New research suggests that a star's initial rotation speed explains why some repeating tidal disruption events produce progressively fainter light flares.
Key facts
- •Repeating partial tidal disruption events occur when a star survives multiple close passes by a supermassive black hole.
- •Researchers identified that about four of the 10 known repeating systems exhibit flares that become dimmer over time.
- •The study suggests that stars in these systems are likely tidally locked and rotating rapidly before their first encounter.
- •The Hills mechanism explains how a binary star system is separated by a black hole, resulting in the capture of a rapidly spinning star.
- •The findings may help explain the origins of some stars orbiting Sagittarius A* at the center of the Milky Way.
Astrophysicists at Syracuse University have identified why some repeating partial tidal disruption events (rpTDEs) produce flares that become dimmer with each encounter. While previous models struggled to explain this fading, the team found that if a star is already spinning rapidly before its first encounter with a supermassive black hole, it does not increase its rotation speed as much during subsequent passes, leading to a decrease in the peak fallback rate of stellar material.
The Mystery of Fading Flares
In a repeating partial tidal disruption event, a star survives multiple close passes by a supermassive black hole, shedding mass and creating a burst of light each time. Astronomers have observed that in about four of the roughly 10 known repeating systems, these flares become progressively dimmer. Previous simulations suggested that even with less material stripped from the star, the peak brightness of the flares should remain relatively consistent due to the black hole's torque increasing the star's rotation speed.
The Role of Initial Stellar Rotation
The research team, led by doctoral student Ananya Bandopadhyay, found that a star's pre-encounter rotation is the key factor. If a star is already rotating rapidly, the tidal forces from the black hole cannot spin it up significantly more during later passages. This stability in rotation means that as the star loses less mass over time, the rate at which material falls back to the black hole decreases, resulting in the observed fading flares.
The Hills Mechanism
The study proposes that the Hills mechanism may explain both the rapid initial rotation and the tight orbits of these stars. In this scenario, a binary star system approaches a supermassive black hole, which tears the pair apart, capturing one star into a tight orbit. Because the stars in such a compact binary would be tidally locked, the captured star would already be spinning rapidly before its first encounter with the black hole.
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This article was independently rewritten by ManyPress editorial AI from reporting originally published by ScienceDaily.
