This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: At the center of most galaxies lies a supermassive black hole, with a mass millions to billions of times that of our sun and some of the most extreme gravity in the universe. Some stars that venture too close to such black holes live to tell the tale.
Rather than being completely destroyed, they survive to make repeated close passes, producing a new flare of light each time. These repeating partial tidal disruption events (rpTDEs) give astronomers the opportunity to watch the same star and black hole interaction unfold again and again, thanks to wide-field time-domain surveys that repeatedly scan large areas of the sky for objects that change in brightness. But in several cases, researchers have noticed a puzzling pattern: The successive flares grow progressively dimmer.
For years, theoretical models couldn't explain why. Now, a team of astrophysicists at Syracuse University has shown that the answer may lie in a previously overlooked factor—the star's spin. The study, published in The Astrophysical Journal, 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.
In a typical tidal disruption event (TDE), a black hole's tidal force—the difference in gravitational pull across a nearby star—tears the star completely apart. As the disrupted stellar debris falls toward, or "accretes" onto, the black hole, it loses energy that is emitted in the form of light over the course of days to months. While black holes themselves emit no light, TDEs provide a short-lived supply of fuel that lights up the surrounding region, allowing astronomers to study these otherwise invisible objects indirectly.
If a star orbiting a black hole does not come close enough to be completely ripped apart, it can, however, lose a fraction of its mass, resulting in a partial TDE. In a repeating partial TDE, the surviving core continues orbiting the black hole, losing more material with each new close pass, a few months to several years apart. How much material a star loses during repeated encounters depends partly on its internal structure.
Bandopadhyay compares a low-mass star to a fluffy meringue, which can become increasingly vulnerable to the black hole's tidal forces. By contrast, a higher-mass star has a more centrally concentrated, onion-like structure and can shed its outer layers while its dense core remains relatively unaffected, losing decreasing amounts of mass over time. Those differences can help explain why rpTDEs don't all behave the same way.
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