Most galaxies are believed to host a supermassive black hole at their center. These extraordinary objects can be millions or even billions of times more massive than the Sun, creating some of the universe’s most powerful gravitational environments.
When a star passes dangerously close to a supermassive black hole, it is not always destroyed immediately. Some stars survive the encounter and return on highly elliptical orbits, producing new bursts of light each time they approach the black hole.
These events are known as repeated partial tidal disruption events (rpTDEs). They allow astronomers to observe the same star interacting with the same black hole multiple times. Wide-field time-domain surveys make these observations possible by repeatedly scanning large regions of the sky and tracking objects that change in brightness.
However, some of these systems present a major mystery. Instead of producing flares with similar brightness during each return, their emissions gradually become fainter. For years, theoretical models have struggled to explain this fading behavior.
A new study by Syracuse University astrophysicists suggests that a previously overlooked property of the star—how rapidly it was rotating before its first close encounter with the black hole—may provide the answer.
This research, published in The Astrophysical Journal, was led by doctoral student Ananya Bandopadhyay in collaboration with postdoctoral fellow Benjamin Armend, associate professor Eric Coughlin of the Department of Physics, and researchers from other institutions.
How black holes tear apart stars
In a standard tidal disruption event (TDE), a black hole’s gravity varies so dramatically across a nearby star that the star is completely pulled apart.
The resulting stellar debris begins to fall toward, or “accrete” onto, the black hole. As the material loses energy and forms an accretion flow, it releases intense light over a period ranging from days to months.
Black holes themselves do not emit light, but tidal disruption events provide a temporary supply of material that illuminates their surroundings. This glow gives astronomers an indirect way to study otherwise invisible black holes.
Not every close encounter completely destroys the star. If a star passes near a black hole without crossing the threshold for total disruption, it may lose only part of its mass, creating a partial TDE. During repeated partial tidal disruption events, the surviving stellar core remains in orbit and returns for additional close encounters, shedding more material each time. These encounters may occur months or years apart.
Why do some black hole flares keep fading?
The amount of material stripped from a star during repeated encounters depends partly on its internal structure. Bandopadhyay compares low-mass stars to fluffy meringues because they can become increasingly vulnerable to a black hole’s tidal forces.
More massive stars behave differently. Their material is concentrated toward the center, giving them an onion-like internal structure. These dense stellar cores can lose their outer layers while remaining largely intact, causing the amount of mass stripped away to decrease during successive encounters.
These structural differences help explain why repeated partial tidal disruption events do not all evolve in the same way. However, one observation has been especially difficult to explain. Of the approximately 10 repeating systems identified so far, four show flares that gradually fade.
It may seem natural to assume that less stripped material would produce a weaker flare. Yet previous fluid-dynamics simulations revealed a complication: even when the star lost less material during each passage, the models still predicted flares with nearly the same peak brightness.
“We were puzzled by this for two years,” Bandopadhyay said.
Their previous work revealed another important consequence of a black hole’s tidal forces. These forces not only pull material away from the star but also exert torque, causing the star to spin faster after each close encounter.
As the star’s rotation increases, the rate at which stripped material returns to the black hole changes. Although less material falls back, it returns more quickly. This concentrated flow can maintain similar peak fallback rates and therefore produce flares with approximately the same predicted brightness.
A rapidly rotating star changes the situation
To reproduce the fading flares observed by astronomers, the researchers introduced what Bandopadhyay called a “new component”: a star that was already rotating rapidly before its first encounter with the black hole.
New simulations suggest that a rapidly rotating star cannot speed up significantly during subsequent passages. If its rotation does not increase substantially after each encounter, the time required for stripped material to fall back toward the black hole remains relatively stable.
This changes the outcome. As progressively less material is removed from the star, the peak fallback rate also decreases. The resulting flares become fainter after each encounter, potentially matching the observations.
How was this star captured?
The discovery raises another question: Why would a star approaching a supermassive black hole already be spinning so quickly?
“It is also extremely difficult to tightly ‘couple’ a star to a supermassive black hole so that it can orbit the black hole in a matter of months, but rpTDEs appear to be able to do that,” Coughlin said.
A process known as the Hills mechanism may explain both the star’s rapid rotation and its unusually tight orbit.
In this scenario, two stars orbiting one another closely approach a supermassive black hole. The black hole’s gravity tears the binary system apart: one star is ejected, while the other is captured into orbit around the black hole.
Stars in extremely close binary systems can become tidally locked, meaning each star rotates on its axis at the same rate that the pair orbits one another. The tighter the binary, the shorter its orbital period—and the faster a tidally locked star must rotate.
The original binary system must be highly compact to leave one star trapped in the short orbit observed in rpTDE systems. That same tight configuration would naturally produce a rapidly rotating, tidally locked star before the black hole captured it.
“Ananya’s work shows 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 its stars,” Coughlin said. “From a theoretical perspective, this is a major step forward in understanding the physics at play in these systems.”
Possible connection with the Milky Way
The implications may extend beyond distant systems with repeating, fading flares. Coughlin notes that some stars currently orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way, may also have been captured through the Hills mechanism.
If so, the same process that may explain the fading flares of repeated partial tidal disruption events could also help astronomers understand unusual stellar populations surrounding black holes in our own galaxy—what Coughlin describes as “our own cosmological backyard.”
Source: www.sciencedaily.com