Astronomers have discovered a star that survived being swallowed by a black hole

When black holes swallow down massive amounts of matter from the space around them, they're not exactly subtle about it. They belch out tremendous flares of X-rays, generated by the material heating to intense temperatures as it's sucked towards the black hole, so bright we can detect them from Earth.

This is normal black hole behaviour. What isn't normal is for those X-ray flares to spew forth with clockwork regularity, a puzzling behaviour reported in 2019 from a supermassive black hole at the centre 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 UK identified a potential cause - a dead star that's endured its brush with a black hole, trapped on a nine-hour, elliptical orbit around it. Every close pass, or periastron, the black hole slurps up more of the star's material.

"This white dwarf is locked into an elliptical orbit close to the black hole, orbiting every nine hours," King explained back in April 2020.

"At its closest approach, about 15 times the radius of the black hole's event horizon, gas is pulled off the star into an accretion disk around the black hole, releasing X-rays, which the two spacecraft are detecting."

The black hole is the nucleus of a galaxy called GSN 069, and it's pretty lightweight as far as supermassive black holes go - only 400,000 times the mass of the Sun. Even so, it's active, surrounded by a hot disc of accretion material, feeding into and growing the black hole.

According to King's model, this black hole was just hanging out, 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 divested the star of its outer layers, speeding its evolution into a white dwarf, the dead core that remains once the star has exhausted its nuclear fuel (white dwarfs shine with residual heat, not the fusion processes of living stars).

But rather than continuing on its journey, the white dwarf was captured in orbit around the black hole, and continued to feed into it.

Based on the magnitude of the X-ray flares, and our understanding of the flares that are produced by black hole mass transfer, and the star's orbit, King was able to constrain the mass of the star, too. He calculated that the white dwarf is around 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 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 ago run out of hydrogen.

"It's remarkable to think that the orbit, mass and composition of a tiny star 250 million light years away could be inferred," King said.

Based on these parameters, he also predicted that the star's orbit wobbles slightly, like a spinning top losing speed. This wobble should repeat every two days or so, and we may even be able to detect it, if we observe the system for long enough.

This could be one mechanism whereby black holes grow more and more massive over time. But we'll need to study more such systems to confirm it, and they may not be easy to detect.

For one, GSN 069's black hole is lower mass, which means that the star can travel on a closer orbit. To survive a more massive black hole, a star would have to be on a much larger orbit, which means any periodicity in the feeding would be easier to miss. And if the star were to stray too close, the black hole would destroy it.

But the fact that one has been identified offers hope that it's not the only such system out there.

"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 this one, there may be many more that we are missing elsewhere in the Universe," King said.

As for the star's future, well, if nothing else is to change, the star will stay right where it is, orbiting 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 a little bigger than Earth - until it's down to a planetary mass, maybe even eventually turning into a gas giant.

"It will try hard to get away, but there is no escape," King said. "The black hole will eat it more and more slowly, but never stop."

The research has been published in the Monthly Notices of the Royal Astronomical Society.

A version of this article was first published in April 2020.

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