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When black holes swallow huge amounts of matter from the space around them, they’re not exactly subtle about it. They spit out huge flares of x-rays, generated by the material heating to intense temperatures as it is sucked into the black hole, so bright we can detect them from Earth.
This is normal behavior of the black hole. What’s not normal is that these x-ray flares go off with mechanical regularity, a confusing behavior reported in 2019 from a supermassive black hole at the center of a galaxy 250 million light years away. . Every nine hours, boom – x-ray rash.
After careful study, astronomer Andrew King of the University of Leicester in the UK identified a potential cause – a dead star that suffered its rubbing with a black hole, trapped in a nine-hour elliptical orbit around she. With each close passage, or periastron, the black hole sucks more material from the star.
“This white dwarf is locked in an elliptical orbit close to the black hole, orbiting every nine hours,” King explained in April 2020.
“At its closest approach, about 15 times the radius of the black hole’s event horizon, gas is pulled from the star into an accretion disk around the black hole, releasing x-rays, which both spaceships detect. “
The black hole is the nucleus of a galaxy called GSN 069, and it is quite light when it comes to supermassive black holes – only 400,000 times the mass of the Sun. Even so, it is active, surrounded by a hot disk of accretion material, fueling and expanding the black hole.
According to King’s model, this black hole was just hanging around, accreting actively, when a red giant star – the last stages of evolution of a sun-like star – ventured a little too close.
The black hole quickly stripped the star of its outer layers, accelerating its evolution into a white dwarf, the dead core that remains after the star has used up its nuclear fuel (white dwarfs glow with waste heat, not fusion process of living stars).
But rather than continue its journey, the white dwarf was captured orbiting the black hole and continued to feed there.
Based on the magnitude of the x-ray flares and our understanding of the flares produced by mass transfer from black holes and the star’s orbit, King was also able to constrain the star’s mass. He calculated that the white dwarf is about 0.21 times the mass of the Sun.
While on the lighter end of the scale, that’s a pretty standard mass for a white dwarf. And if we assume that the star is a white dwarf, we can also infer – based on our understanding of other white dwarfs and stellar evolution – that the star is rich in helium, having long been short of of hydrogen.
“It is remarkable to think that the orbit, mass and composition of a small star 250 million light years away could be inferred,” King said.
Based on these parameters, he also predicted that the star’s orbit would oscillate slightly, like a spinning top losing speed. This oscillation should repeat itself every two days or so, and we may even be able to detect it if we observe the system long enough.
This could be a mechanism by which black holes become more and more massive over time. But we will have to study more of such systems to confirm this, and they may not be easy to detect.
On the one hand, the GSN 069 black hole is of lower mass, which means that the star can travel in a closer orbit. To survive a more massive black hole, a star would have to be in a much larger orbit, meaning any periodicity in the feed would be easier to miss. And if the star moved too close, the black hole would destroy it.
But the fact that such a system has been identified gives hope that it is not the only system of this type.
“In astronomical terms, this event is only visible to our current telescopes for a short time – around 2000 years, so unless we have been extraordinarily lucky to have caught this one, there may be some. many more that we miss elsewhere in the Universe, ”King said.
As for the future of the star, well, if nothing else needs to change, the star will remain where it is, orbiting the black hole, and continuing to be slowly stripped for billions of years. years. This will cause it to grow larger and decrease in density – white dwarfs are only a little larger than Earth – until it becomes a planetary mass, perhaps even ends up turning into a gas giant.
“He will do his best to get away, but there is no escape,” King said. “The black hole will eat it slower and slower, but never stop.”
The research was published in the Monthly notices from the Royal Astronomical Society.
A version of this article was first published in April 2020.
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