The eclipsed star has had multiple close encounters with a supermassive black hole in a distant galaxy and may have survived material torn apart by giant gravitational tidal forces.
destruction a star by the gravitational forces of a supermassive black hole is a violent event known as a tidal disruption event (TDE). The gas is torn away from the star and undergoes “spaghettiization,” in which it is shredded and stretched into streams of hot material flowing around it. black hole, forming a temporary and very bright accretion disk. In our opinion, central galaxy appears to host a supermassive black hole.
On September 8, 2018, the All-Sky Automated Survey for Supernovae (ASASSN) detected a flare at the core of a distant galaxy 893 million light-years away. Cataloged as AT2018fyk, the flare had all the hallmarks of a TDE. Various X-ray telescopes, including NASA Quickof Europe XMM-Newtonthe MORE BEAUTIFUL Instrument installed on the International Space Station and Germany EROSITE, observed the black hole glow dramatically. Typically, TDEs exhibit a smooth decline in brightness over several years, but when astronomers revisited AT2018fyk about 600 days after it was first noticed, the X-rays quickly disappeared. Even more mysteriously, about 600 days later, the black hole suddenly flared up again. What was going on?
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“Until now, the assumption has been that when we see the results of a close encounter between a star and a supermassive black hole, the result will be fatal for the star; that is, the star is completely destroyed,” said Thomas Wevers, an astronomer at the European Southern Observatory and author of a new study on the event. , a statement. “But unlike all the other TDEs we know of, when we pointed our telescopes back at the same spot a few years later, we saw it light up again.”
Wevers led a team of astronomers who realized that the repeated flares were the signature of a star that had survived a TDE and completed another orbit to experience a second TDE. To fully explain what they observed, Wevers’ group developed a “repetitive partial TDE” model.
In their model, the star was once a member of a binary system passed very close to the black hole at the center of its galaxy. The black hole’s gravity pulled one of the stars away and it became a runaway star. hypervelocity star race through the galaxy at 600 miles (1,000 kilometers) per second. The other star was tightly bound to the black hole in a 1,200-day elliptical orbit that took it to what scientists call its tidal radius—the distance from the black hole at which the star begins to be torn apart by gravitational tides. black hole.
Because the star is not exactly within the tidal radius, only a fraction of its material has been detached, leaving a dense stellar core that continues to orbit the black hole. It takes about 600 days for a black hole to form an accretion disk of material pulled from a star, so when astronomers saw the system flare, it was safely near the farthest point of the star’s orbit.
But as the star’s core began to approach the black hole again, about 1,200 days after its first encounter, the star began to steal some of its material back from the accretion disk, causing the X-rays to suddenly fade. “When the nucleus returns to the black hole, it gravitationally steals all the gas from the black hole, and as a result, there is no matter left to accumulate, and therefore the system darkens,” said Dheeraj Pasham, co-author of the study. Research and an astrophysicist from MIT, in a comment.
But a black hole weight soon returns the favor by stealing more material as the star approaches. As with the initial encounter, the black hole has a 600-day delay from its snack on the star to forming an accretion disk, which explains why the X-rays are triggered again.
From the star’s orbit, Wevers’ team calculated that the black hole’s mass is about 80 million times that of our sun, or about 20 times that of the black hole at the center of our planet. Milky way galaxy, Sagittarius A*.
Vevers’ team won’t have to wait long to find out if the theory is true. Scientists predict that AT2018fyk should dim again in August when the star’s core returns to its orbit, and brighten again in March 2025 when new material begins to accumulate into the black hole.
However, there is one potential complication in the amount of mass a star loses to a black hole. The amount of mass lost depends in part on how fast the star is spinning, and the black hole can affect this. If the star spins fast enough to break apart, the black hole will steal material more easily, increasing its mass loss.
“If the mass loss is only on the order of 1%, then we expect the star to survive more encounters, whereas if it is closer to 10%, the star may already be destroyed,” co-author Eric Coughlin of Syracuse University in New York said in a statement about the study.
Regardless, TDEs and recurring partial TDEs provide a rare window into the lives of supermassive black holes that we cannot detect because they are in unusual conditions. This is important for measuring their mass and determining something about how black holes evolve, and therefore how the galaxy around the black hole has evolved over cosmic history.
The findings were presented and published at the 241st meeting of the American Astronomical Society Astrophysical Journal Lettersboth on January 12.
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