A captured star has experienced multiple close encounters with a supermassive black hole in a distant galaxy — and may even have survived as material was ripped away by immense tidal forces.
The destruction of a old by the gravitational forces of a supermassive black hole is a violent affair known as the Tidal Disruption Event (TDE). Gas is ripped from the star and undergoes “spaghettification,” where it is crushed and stretched into streams of hot material that flow around the star black hole, forming a temporary and very bright accretion disk. In our view, the center is the galaxy hosting the supermassive black hole appears to be flickering.
On Sept. August 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 torch had all the hallmarks of a TDE. Various X-ray telescopes, including those of NASA FastEurope XMM Newtonthe KIND Instrument mounted on the International Space Station and Germany eROSITAwatched the black hole brighten dramatically. Normally, TDEs show a smooth decay in brightness over several years, but when astronomers looked again at AT2018fyk about 600 days after it was first observed, the X-rays were quickly gone. Even more puzzling is that the black hole suddenly flared up again about 600 days later. What happened?
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“Previously it was assumed that if we see the aftermath of a close encounter between a star and a supermassive black hole, the outcome will be deadly for the star; that is, the star is completely destroyed,” says Thomas Wevers, an astronomer at the European Southern Observatory and author of new research on the event, said in a expression. “But unlike all other TDEs we know of, when we pointed our telescopes back at the same spot a few years later, we found that it had brightened again.”
Wevers led a team of astronomers who realized 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 model of a ‘repeated partial TDE’.
In her model, the star was once a member of a binary system that passed too close to the black hole at the center of its galaxy. The black hole’s gravity threw off one of the stars, which turned into an outlier hyperspeed star Race out of the galaxy at 600 miles (1,000 kilometers) per second. The other star became tightly bound to the black hole on a 1,200-day elliptical orbit that took it toward the tidal radius — the distance from the black hole at which a star begins to be torn apart by the gravitational tides emanating from the black hole Hole.
Because the star was not entirely within the tidal radius, only some of its matter was eroded, leaving a dense stellar core that continued its orbit around the black hole. It takes about 600 days for material pulled out of the star by the black hole to form the accretion disk. So when astronomers saw the system’s flare, the star was safe, near the farthest point of its orbit.
But as the star’s core approached the black hole again, about 1,200 days after its initial encounter, the star began stealing some of its material back from the accretion disk, causing the X-ray emission to suddenly fade. “When the core returns to the black hole, it essentially steals all the gas out of the black hole by gravity, and as a result there is no matter to accumulate and hence the system becomes dark,” says Dheeraj Pasham, a The study’s co-author and an astrophysicist at MIT said in the statement.
But the black hole heaviness soon returns the favor and steals more material as the star approaches. As with the first encounter, there is a 600-day delay from the black hole snacking on the star to the formation of the accretion disk, which explains why the X-ray flare was turned back on when it did.
From the star’s orbit, Wevers’ team calculated that the black hole has a mass nearly 80 million times that of our Sun, or about 20 times the mass of the black hole at the center of our Sun Milky Way, Sagittarius A*.
Wevers’ team doesn’t have to wait long to find out if the theory is correct. The scientists predict that AT2018fyk should dim again in August, as the star’s core returns, and brighten again in March 2025, when new material begins to accrete onto the black hole.
However, there is a potential complication in the amount of mass the star lost to the black hole. The amount of mass lost depends in part on how fast the star is spinning, which could affect the black hole. If the star spins almost fast enough to break up, the black hole will steal material more easily and increase mass loss.
“If the mass loss is only at the 1% level, then we expect the star to survive many more encounters, whereas if it’s closer to 10%, the star may already have been destroyed,” says Eric Coughlin, a Co -Author on the study from Syracuse University in New York, the statement said.
Regardless, TDEs and repetitive partial TDEs offer a rare window into the life of supermassive black holes that we typically cannot see because they are dormant. This is important to measure their mass and to determine something about how the black holes evolved, and by extension, how the galaxy around the black hole evolved throughout cosmic history.
The results were presented at the 241st meeting of the American Astronomical Society and published in The Letters of the Astrophysical Journalboth on 1. 12.
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