If black holes surround massive amounts of matter from space, it is not exactly subtle about it. They bubble enormous torches of X-rays, generated by the material that heats to intense temperatures while sucking into the black hole, so bright that we can detect it from Earth.
This is normal behavior of black holes. What is not normal is that the X-ray torches erupt at clock speed, a mysterious behavior reported in 2019 from a supermassive black hole in the center of a galaxy 250 million light-years away. Every nine hours, tree – X-ray flare.
After careful study, astronomer Andrew King of the University of Leicester in the United Kingdom identified a possible cause – a dead star that endured its brush with a black hole trapped in a nine-hour elliptical orbit around it. Each short pass, or periastron, creeps up the black hole more of the star’s material.
“This white dwarf is trapped in an elliptical orbit near the black hole, which orbits every nine hours,” King explained in April 2020.
“With its closest approximation, about 15 times the radius of the black hole’s event horizon, gas is subtracted from the star in a growth disk around the black hole, releasing X-rays that detect the two spacecraft.”
The black hole is the core of a galaxy called GSN 069, and it is fairly lightweight in terms of supermassive black holes – only 400,000 times the mass of the sun. Nevertheless, it is active, surrounded by a hot disk of accumulating material that enters the black hole and grows.
According to King’s model, this black hole just hung out and made its active growth when a red giant star – the final evolutionary stages of a sunny star – accidentally wandered too close.
The black hole immediately deposited the star from its outer layers and accelerated its evolution into a white dwarf, the dead nucleus that remained as soon as the star depleted its nuclear fuel (white dwarfs shine with residual heat, not the fusion processes of living stars not).
But rather than continue his journey, the white dwarf was caught in an orbit around the black hole and continued in it.
Based on the magnitude of the X-rays and our understanding of the torches produced by the mass transmission of black holes, 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 is a fairly standard mass for a white dwarf. And if we assume that the star is a white dwarf, we can also conclude from our understanding of other white dwarfs and star evolution that the star is rich in helium, because it no longer needs hydrogen.
“It’s remarkable to think that the orbit, mass and composition of a small star can be deduced 250 million light – years away,” King said.
Based on these parameters, he also predicted that the star’s orbit would sway slightly, like a rotating speed losing. This pendulum must be repeated every two days, and we can even detect it if we keep an eye on the system long enough.
This may be one mechanism by which black holes become larger over time. But we will have to study more such systems to confirm this, and it may not be easy to detect.
GSN 069’s black hole has lower mass, which means that the star can move on a narrower orbit. To survive a massive black hole, a star must be on a much larger orbit, meaning any periodicity in the feed would be easier to miss. And if the star wandered too close, the black hole would destroy it.
But the fact that one has been identified hopes that it is not the only system.
“Astronomically, this event has only been visible for a short time to our current telescopes – about 2,000 years, so unless we were extraordinarily lucky to catch one, there may be many more that we miss elsewhere in the universe.” King said.
As for the future of the star, yes, if nothing else wants to change, the star will just stay there, orbit the black hole and 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 leads to a planetary mass, and may even eventually become a gas giant.
“It will try hard to get away, but there is no escape,” King said. “The black hole eats it more and more slowly, but never stops.”
The research was published in the Monthly notices from the Royal Astronomical Society.
A version of this article was first published in April 2020.