When black holes swallow massive amounts of matter from the space around them, they aren’t exactly subtle about it. They are blasting out massive bursts of X-rays generated by heating the material to intense temperatures as it is sucked into the black hole, so bright that we can detect them from Earth.
This is normal behavior of a black hole. What’s not normal is that those X-ray flares are spewed out clockwise, 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 possible cause: a dead star that has passed its brush with a black hole trapped in a nine-hour elliptical orbit around it. Each narrow pass, or periastron, absorbs more of the star’s material.
“This white dwarf is trapped in an elliptical orbit close to the black hole and orbits every nine hours,” King explained in April 2020.
“At the closest approach, about 15 times the radius of the black hole’s event horizon, gas is drawn from the star to an accretion 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’s quite light as far as supermassive black holes go – just 400,000 times the mass of the sun. Yet it is active, surrounded by a hot disk of accretion material that feeds and grows the black hole.
According to King’s model, this black hole was just hanging around and doing its active accretion thing, when a red giant star – the final evolutionary stages of a sun-like star – happened to wander a little too close.
The black hole promptly stripped the star of its outer layers, accelerating its evolution into a white dwarf, the dead core that remains once the star has exhausted its nuclear fuel (white dwarfs shine with waste heat, not the fusion processes of living stars) .
But instead of continuing its journey, the white dwarf was caught in orbit around the black hole and kept feeding in it.
Based on the size 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 limit the star’s mass. He calculated that the white dwarf is about 0.21 times heavier than the sun.
While on the lighter side of the scale, that’s a pretty standard mass for a white dwarf. And if we assume that the star is a white dwarf, then – based on our understanding of other white dwarfs and the evolution of stars – we can also conclude that the star is rich in helium because it ran out of hydrogen a long time ago.
“It is remarkable to think that the orbit, mass and composition of a small star can be inferred 250 million light-years away,” said King.
Based on these parameters, he also predicted that the star’s orbit wobbles a bit, like a spinning top losing that speed. This wobble should be repeated about every two days, and we can even detect it if we observe the system long enough.
This could be a mechanism that causes black holes to get bigger and bigger over time. But we’ll have to study more such systems to confirm it, and they may not be easy to detect.
First, the black hole of GSN 069 has a smaller mass, which means that the star can travel in a shorter orbit. To survive a larger black hole, a star would have to be in a much larger orbit, meaning any dietary periodicity is easier to miss. And if the star strayed too close, the black hole would destroy it.
But the fact that one has been identified gives hope that it isn’t the only such system.
“In astronomical terms, this event is visible to our current telescopes for only a short time – about 2,000 years, so unless we were extremely lucky to capture it, there may be many more that we miss elsewhere in the Universe.” King said.
As for the star’s future, if nothing else changes, the star will stay exactly where it is, orbiting the black hole, and be slowly stripped for billions of years. Because of this, it will grow in size and decrease in density – white dwarfs are only slightly larger than Earth – until it reaches planetary mass, perhaps even eventually turning into a gas giant.
“He’ll do his best to get away, but there’s no getting around it,” said King. “The black hole will eat it slower and slower, but never stop.”
The research is published in the Monthly Communications from the Royal Astronomical Society.
A version of this article was first published in April 2020.