The captured star has experienced several close encounters with a supermassive black hole in a distant galaxy – and may have even escaped shredding matter due to its massive gravitational tidal forces.
destroy a star by the gravitational forces of Giant black hole It is a violent affair known as a tidal disturbance event (TDE). Gas is ripped from the star and undergoes a process of “spagitation,” in which it is slashed and stretched into streams of hot material that flow around Black hole, forming a temporary and very bright accretion disk. From our point of view, the center galaxy The supermassive black hole appears to be glowing.
On September 8, 2018, the All-Sky Automated Survey for Supernovae (ASASSN) detected a glow in the nucleus of a distant galaxy 893 million light-years away. The flare was classified as AT2018fyk, and it had all the hallmarks of a TDE. Various X-ray telescopes, including NASA telescopes Swiftin Europe XMM-Newtonthe Nice Instrument installed on the International Space Station, German AerositaNote that the black hole is shining brightly. Normally, TDEs show a smooth decrease in brightness over several years, but when astronomers looked back at AT2018fyk about 600 days after they were first observed, the X-rays quickly disappeared. Even more puzzling, 600 days after that, the black hole suddenly erupted again. What happened?
“Until now, the assumption has been that when we see the aftermath of a close encounter between a star and a supermassive black hole, the outcome will be fatal for the star; that is, the star is completely destroyed,” said Thomas Weavers, an astronomer at the European Southern Observatory and author of new research on the event. , in a statment. “But unlike all the other TDEs we know of, when we pointed our telescopes at the same location again several years later, we found that they had re-brightened.”
Wevers led a team of astronomers who realized that repeat flares were a signature of a star that survived a TDE and completed another orbit for a second TDE. To fully explain what they were observing, Wevers’ group developed a “partial recurrent TDE” model.
In their model, the star was once a member of the binary system that passed near the black hole at the center of its galaxy. The gravity of the black hole pushed one of the stars away, and it turned into a runaway Speed star Race at 600 miles (1,000 km) per second out of the galaxy. The other star became tightly harnessed to the black hole, in a 1,200-day elliptical orbit that took it toward what scientists call the tidal radius—the distance from the black hole at which the star begins to be torn apart by the gravitational tides emanating from the black hole.
Because the star was not within its tidal radius, only some of its material was stripped away, leaving a dense stellar core that continued in its orbit around the black hole. It takes roughly 600 days for the material the black hole pulls from a star to form an accretion disk, so by the time astronomers saw the system’s glow, the star was safe, near the farthest point in its orbit.
But when the star’s core began to approach the black hole again, about 1,200 days after their first encounter, the star began reclaiming some of its material again from the accretion disk, causing the X-ray emission to suddenly die down. “When the core goes back into the black hole, it basically steals all the gas away from the black hole via gravity, and as a result there is no matter accumulating, so the system becomes dark,” said Dheeraj Pasham, a co-author of the study and an astrophysicist in the journal Science. Massachusetts Institute of Technology, per statement.
But the black hole gravity He quickly returns the favor, and steals more items as the star approaches. As happened during the initial encounter, there is a 600-day lag from the black hole snacking on the star to the formation of the accretion disk, which explains why the X-ray glow restarted when that happened.
From the star’s orbit, Wavers’ team estimated the black hole to be about 80 million times the mass of our Sun, or about 20 times the mass of the black hole at the center of our planet. Milky WayAnd arch a*.
Weavers’ team won’t have to wait long to see if the theory turns out to be true. Scientists expect AT2018fyk to darken again in August, when the star’s core turns back on, and to get brighter again in March 2025 when new material begins accreting onto the black hole.
However, there is a potential complication in how much mass the star has lost to the black hole. The amount of mass lost depends in part on the speed of the star’s rotation, which the black hole may be affecting. If the star was spinning nearly fast enough to break apart, the black hole would easily steal material, adding to the mass loss.
“If the mass loss is only at the 1% level, we would expect the star to survive many more encounters, whereas if it is closer to 10%, the star may have already been destroyed,” Eric Coughlin, co-author on the study from Syracuse University In New York, he said in the statement.
Regardless, repeated partial TDEs and TDEs provide a rare window into the life of supermassive black holes that we normally can’t detect because they’re asleep. This is important for measuring their mass and determining something about how black holes evolved, and then how the galaxy around the black hole also evolved throughout cosmic history.
The results were presented at the 241st meeting of the American Astronomical Society and published in Astrophysical Journal Lettersboth on January 12.
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