When black holes swallow massive amounts of matter from the space around them, they are not exactly subtle. These erupt extraordinary X-ray flames, generated by the heating of the material at intense temperatures, as it is sucked into the black hole, so bright that we can detect them from Earth.
This is a normal black hole behavior. What is not normal is for those X-ray missiles to throw with mechanical regularity, a puzzling 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-rays.
After careful study, astronomer Andrew King of the University of Leicester in the UK identified a potential cause – a dead star enduring its brush with a black hole, trapped in a nine-hour elliptical orbit around it. With each passing pass, or periastron, the black hole seeps more of the star’s material.
“This white dwarf is trapped in an elliptical orbit near the black hole, orbiting every nine hours,” King explained in April 2020.
“At the closest approach, about 15 times the radius of the black hole’s event horizon, the gas is drawn from the star into an accumulation disk around the black hole, releasing X-rays, which the two spacecraft detect.”
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 larger than the mass of the Sun. Even so, it is active, surrounded by a hot disk of accumulation material, which feeds and grows the black hole.
According to King’s model, this black hole had just come out, doing its active accretion activity, when a giant red star – the final evolutionary stages of a Sun-like star – happened to wander a little too close.
The black hole promptly removed the star from its outer layers, accelerating its evolution into a white dwarf, the dead core that remains once the star has depleted its nuclear fuel (white dwarfs glow with residual heat, not the fusion processes of living stars).
But rather than continue its journey, the white dwarf was captured in orbit around the black hole and continued to feed in it.
Based on the size of the X-ray missiles and our understanding of the missiles produced by the mass transfer of the black hole and the orbit of the star, King also managed to constrain 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, this is a fairly standard table 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, having long since run out of hydrogen.
“It’s remarkable to think that the orbit, mass, and composition of a small star 250 million light-years away could be deduced,” King said.
Based on these parameters, he also predicted that the star’s orbit would sway slightly, like a spinning top that loses speed. This movement should be repeated 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 grow more and more over time. But we will have to study several such systems to confirm it and it may not be easy to detect.
First, the black hole of the GSN 069 has a smaller mass, which means that the star can travel in a closer orbit. To survive a more massive black hole, a star should be in a much larger orbit, which means that any periodicity in feeding would be easier to miss. And if the star got too close, the black hole would destroy it.
But the fact that one has been identified gives hope that it is not the only such existing system.
“In astronomical terms, this event is only visible to our current telescopes for a short period of time – about 2,000 years, so if we weren’t extraordinarily lucky to catch it, there could be many more that we’re missing. in another part of the Universe ”. King said.
As for the star’s future, well, if nothing else changes, the star will remain exactly where it is, orbiting the black hole and continuing to be slowly undressed for billions of years. This will cause it to increase in size and decrease in density – white dwarfs are only slightly larger than Earth – until it reaches a planetary mass, maybe even turning into a gas giant.
“They will try hard to escape, but there is no escape,” King said. “The black hole will eat it more and more slowly, but it will never stop.”
The research was published in Monthly notifications from the Royal Astronomical Society.
A version of this article was first published in April 2020.