When black holes swallow large amounts of matter from the space around them, they are not exactly subtle. They expel huge flames of X-rays, generated by the heating of the material at intense temperatures while sucking towards the black hole, so bright that we can detect them from Earth.
This is a normal black hole behavior. What’s not normal is that these X-ray flares launch regularly from the clock, a baffling behavior reported in 2019 from a supermassive black hole in the center of a galaxy 250 million light-years away. Every nine hours, boom: X-ray flare.
After careful study, astronomer Andrew King of the University of Leicester in the United Kingdom identified a potential cause: a dead star holding the brush with a black hole, trapped in a nine-hour elliptical orbit at around him. At each nearby step, or periastron, the black hole shines more than the material of the star.
“This white dwarf is locked in an elliptical orbit near the black hole, orbiting every nine hours,” King explained in April 2020.
“At its closest point, approximately 15 times the radius of the black hole event horizon, the gas is removed from the star in an accretion disk around the black hole, releasing X-rays that the two spacecraft are detecting “.
The black hole is the core of a galaxy called GSN 069, and is quite light in terms of supermassive black holes, only 400,000 times the mass of the Sun. However, it is active, surrounded by a hot disk of accretion material, which feeds and grows the black hole.
According to King’s model, this black hole had just hung, making its active accretion, when a red giant star – the final evolutionary stages of a Sun-like star – passed by a little too close.
The black hole quickly undid the star from its outer layers, accelerating its evolution into a white dwarf, the dead nucleus remaining once the star has depleted the nuclear fuel (white dwarfs glow with residual heat, no the processes of fusion of living stars).
But instead of continuing its journey, the white dwarf was captured in orbit around the black hole and continued to feed on it.
Based on the magnitude of the X-ray lights and our understanding of the lights produced by the mass transfer of the black hole and the orbit of the star, King was also able to limit the mass of the ‘star. He calculated that the white dwarf is about 0.21 times the mass of the Sun.
While at the lighter end of the scale, it’s a pretty standard dough for a white dwarf. And if we assume that the star is a white dwarf, we can also deduce, based on our understanding of other white dwarfs and stellar evolution, that the star is rich in helium since time immemorial. which has run out of hydrogen.
“It’s remarkable to think that the orbit, mass, and composition of a small star could be inferred 250 million light-years away,” King said.
Based on these parameters, he also predicted that the star’s orbit would oscillate slightly, as if a rotation were losing speed. This oscillation should be repeated every two days or so, and we can even detect it if we observe the system for a sufficient time.
This could be one of the mechanisms by which black holes grow more and more massively over time. But we will have to study more such systems to confirm this and they may not be easy to detect.
On the one hand, the black hole of GSN 069 has a 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 that any periodicity in the feed would be easier to lose. And if the star went too far, the black hole would destroy it.
But the fact that it has been identified offers hope that it is not the only system of its kind.
“In astronomical terms, this event is only visible to our current telescopes for a short time: about 2,000 years, so unless we were extraordinarily lucky to have caught it, we may find many more missing in anywhere else in the universe. ” “King said.”
As for the future of the star, well, if nothing else changes, the star will remain right where it is, orbiting around the black hole and continuing to be slowly stripped for billions of years. This will cause it to grow in size and decrease in density (white dwarfs are only slightly larger than Earth) until it is reduced to a planetary mass, perhaps even becoming a gas giant.
“It will take a lot of effort to get out of it, but there is no leak,” King said. “The black hole will eat it more and more slowly, but it will never stop.”
The research has been published in Monthly notices from the Royal Astronomical Society.
A version of this article was first published in April 2020.