A captured star has had several close encounters with a supermassive black hole in a distant galaxy – and may even have survived material torn apart by immense gravitational tidal forces.
The destruction of a Star by the gravitational forces of a a supermassive black hole is a violent affair known as a Tidal Disturbance Event (TDE). The gas is stripped from the star and undergoes “spaghettification”, during which it is shredded and stretched into streams of hot material that circulate around the star. black hole, forming a temporary and very bright accretion disk. From our point of view, the center of the galaxy housing the supermassive black hole appears to be igniting.
On September 8, 2018, the All-Sky Automated Survey for Supernovae (ASASSN) spotted an eruption in the core of a distant galaxy 893 million light-years away. Cataloged as AT2018fyk, the rocket had all the characteristics of a TDE. Various X-ray telescopes, including NASA’s FastEurope XMM-Newtonthem NICER instrument mounted on the International Space Station, and Germany eROSITA, observed the black hole brighten dramatically. Usually, TDEs show a steady dip in brightness over several years, but when astronomers looked at AT2018fyk again about 600 days after it was first noticed, the X-rays had quickly disappeared. Even more puzzling, about 600 days after that, the black hole suddenly exploded again. What was happening?
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“Until now, the assumption has been that when we see the consequences 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 Wevers, an astronomer at the European Southern Observatory and author of new research on the event, said in a statement. “But unlike every other TDE we know of, when we pointed our telescopes again at the same spot several years later, we found it had reignited.”
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 were observing, Wevers’ group developed a model of “repetitive partial TDE”.
In their model, the star was once a member of a binary system passing too close to the black hole at the center of its galaxy. The black hole’s gravity threw one of the stars away, which turned into a runaway hyperspeed star run at 600 miles (1,000 kilometers) per second out of the galaxy. The other star became tightly bound to the black hole, in a 1,200-day elliptical orbit that brought it toward what scientists call the tidal radius – the distance from the black hole at which a star begins to tear apart. the gravitational tides emanating from the black hole.
Because the star was not entirely within the 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 about 600 days for material extracted from the star by the black hole to form the accretion disk, so by the time astronomers saw the system go up in flames, the star was safe, near the point the farther from its 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 pick up some of its material from the accretion disk, causing a sudden decrease in the emission of X-rays.” When the nucleus returns to the black hole, it essentially steals all the gas from the black hole by gravity, and therefore there is no matter to accrete and therefore the system shuts down “, Dheeraj Pasham, co-author on the study and astrophysicist at MIT, said in the press release.
But the black hole is gravity soon returns the favor, stealing more material from the star’s close approach. As with the initial encounter, there is a 600-day lag between the star-munching black hole and the formation of the accretion disk, explaining why the X-ray flare reignited at that time.
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 more massive than the black hole at the center of our planet. milky way galaxy, Sagittarius A*.
Wevers’ team won’t have to wait long to find out if the theory is correct. Scientists predict that AT2018fyk should darken again in August when the star’s core returns, and brighten again in March 2025 when new material begins to accumulate on 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 how fast the star spins, which the black hole could affect. If the star spins almost fast enough to break up, the black hole will more easily steal material, increasing the loss of mass.
“If the mass loss is only at the 1% level, then we would expect the star to survive for many more encounters, whereas if it is closer to 10%, the star may have -be already destroyed,” said co-author Eric Coughlin. on the study from Syracuse University in New York, said in the statement.
Either way, repeating TDEs and partial TDEs provide a rare window into the life of supermassive black holes that we normally cannot detect because they are dormant. This is important for measuring their mass and determining how black holes have evolved, and therefore how the galaxy around the black hole has also evolved over cosmic history.
The results were presented at the 241st meeting of the American Astronomical Society and published in Letters from the Astrophysical Journalboth on January 12.
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