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 star 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. But one pattern in particular has mystified researchers. Of the roughly 10 repeating systems identified to date, four have produced flares that grow progressively dimmer.
Decreasing mass loss might seem like an obvious explanation. But previous hydrodynamical simulations showed that, surprisingly, even as the material lost decreased with each encounter, the predicted flares retained roughly the same brightness.
“We were puzzled by this for two years,” Bandopadhyay says.
Their previous work had revealed another effect of the black hole's tidal forces.In addition to stripping material from the star, they exert a torque that causes the star to spin faster with each close encounter. As a result, although less material falls back toward the black hole, it returns over a shorter period of time, helping to keep the predicted flare at roughly the same brightness.
To reproduce the dimming astronomers were actually observing, the researchers needed what Bandopadhyay called “a new ingredient”—a star that was already spinning rapidly before its first encounter with the black hole.
The new study found that this initial rotation prevents the star from being significantly spun up during each passage. Without that additional spin-up, the timescale over which the stripped material falls back remains relatively constant. As the star loses less material with each encounter, the peak fallback rate—and the predicted brightness of the flare—can finally decline.
But why would the star already be spinning so rapidly?
“It is also extremely difficult to ‘bind’ a star to a supermassive black hole so tightly that it orbits the black hole in a matter of months, and yet they seem to do so in rpTDEs,” Coughlin says.
The so-called Hills mechanism may explain both. Under this scenario, a pair of closely orbiting stars passes near a supermassive black hole, which tears the binary apart, ejecting one star and capturing the other.
In a close binary, the stars can become tidally locked, causing each to rotate on its axis at the same rate that the pair orbits each other. The tighter the binary, the shorter that orbital period and the faster a tidally locked star spins. The binaries capable of leaving a captured star on the short orbit observed in rpTDEs would have to be extremely tight—also leaving a tidally locked star spinning rapidly before its capture.
“Ananya’s work demonstrates that each of these peculiarities can be explained by the same underlying phenomenon: the tidal destruction of a binary system and the capture of one of the stars,” Coughlin says. “From a theoretical standpoint, this is a major step forward in our understanding of the physics at play in these systems.”
Zooming out, Coughlin notes that Hills capture may also have produced some of the stars orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way. The new findings could therefore help explain some of the properties of stars in what he calls “our own cosmological backyard.”
The Astrophysical Journal