Impossibly Big Black Hole Was Probably Impossible After All

A straightforward error in analysis undermines the entire paper.

A NASA illustration shows what a system containing a black hole and a star might look like. (Image credit: NASA / CXC / M. Weiss)

Two weeks ago (Nov. 27), astronomers published a paper in the journal Nature claiming they'd found an impossibly gigantic black hole not too far from Earth. If they were correct, it would have been a major shock to astrophysics, upending theories of how and where such huge black holes form. But it looks like they were probably wrong.

The researchers thought they'd found the rare, huge black hole, 70 times the mass of our sun, as part of a binary system known as LB-1 that is 15,000 light-years from Earth. But now, two independent papers published to the arXiv database this week found the same basic problem with that claim: It relied on evidence that the unseen black hole was wiggling very slightly as its heavy companion star, known as the B star, wheeled around it. The difference between the black hole's slight wiggle and the star's rapid motion suggested the black hole was much larger — if they were closer to one another's size, you'd expect the black hole to move as much as the star. However, according to the two new papers, the researchers misinterpreted what they were seeing in the light from the distant system.

Picture a sumo wrestler whipping a bowling ball around in circles at the end of a long chain. That's pretty much how the model of this system worked in the Nature paper. The wrestler in that scenario (the black hole) would shift back and forth a little to compensate for the weight of the ball (the companion star), but the ball would do most of the moving. If you knew the mass of the bowling ball and knew how much they were each moving, you could calculate the mass of the sumo wrestler. 

The problem is that the wiggling bit of light the researchers built the claim on — called the "Hα emission line" — now looks like it didn't come from the black hole at all. That means the mind-blowing mass measurement is likely wrong.

Related: 9 Ideas About Black Holes That Will Blow Your Mind

"You've got this high mass 'B star,' and that's one component. And then the black hole is the other component," said Jackie Faherty, an astrophysicist at the American Museum of Natural History in New York City, who wasn't involved in any of these papers. "So you've got these two things that you're looking at but they can get muddled with each other."

Telescopes on Earth generally aren't sharp enough to resolve the individual objects in star systems well enough to measure their movements — especially when one of those objects is a black hole, only visible from the thin "accretion disk" of material around its main body. So studying these systems often requires analyzing the patterns in individual frequencies of light coming from the systems, and using them to draw inferences about what's going on inside them.

LB-1 has one very bright data source: All the light coming off the normal B star in the system. Researchers can measure its movements using the Doppler effect, which makes light wavelengths lengthen and the light appear to redden as the star moves away from Earth, and then get a bit bluer as it moves back toward Earth. Researchers can track that Doppler effect in a series of emission lines — especially bright frequencies of radiation that correspond to individual features of the star.

In the Nature paper, the researchers found another emission line in the system, the Hα line, that didn't seem to come from the normal star. They found it also showed a mild Doppler effect, suggesting its source was moving a little, and hinting that it likely came from the disk of material around an unseen black hole in the system. What the new papers found is that the Nature researchers failed to fully disentangle the data from the bright source, the star, and from the dim source. That apparent wiggling in the Hα line was a sort of illusion created by light from the companion star, and disappears once you properly subtract that source. Whatever's making the Hα line isn't moving at all relative to the system.

"After it's pointed out, it's very easy to understand — it's not something obscure, and I think most astronomers would understand the argument and agree," Leo C. Stein, a University of Mississippi astrophysicist who also wasn't involved in any of these papers, told Live Science.

He said that after seeing the new papers he's "very skeptical" of the initial Nature paper's claim about the black hole's mass.

If the Hα line isn't moving, that means one of two things, University of California, Berkeley, astrophysicists Kareem El-Badry and Eliot Quataert wrote in their paper, one of the two published to arXiv that identified the Hα issue.

"One conceivable interpretation is that the companion is a black hole with even higher mass than reported," they wrote.

Maybe the black hole is so stupendous in size that it doesn't seem to wiggle at all under its companion star's gravitational influence.

"We regard this scenario as exceedingly unlikely," they wrote.

There's no other evidence of such a large black hole in the system.

So the more likely scenario is that the system contains a more typical black hole more or less on the scale of the sun, and the Hα line comes from some other source, as outlined in the second arXiv paper, from a larger team from the Katholieke Universiteit Leuven and the Royal Observatory, both in Belgium.

A third paper, from a team of researchers from New Zealand, Canada, and Australia, identified several more issues with the Nature paper, including that the authors likely misjudged the distance to the system. It's compelling, Stein said, but the Hα issue presents a much more straightforward problem.

The system is still interesting, and El-Badry said in a tweet that he's looking forward to studying it in more detail. But it fits more neatly into existing theories of astrophysics, which easily explains smaller black holes in this region of space, but struggle to explain how a much larger black hole could have formed.

"This is a story of how science progresses," Faherty told Live Science. "Scientists became really intrigued because it was sort of an interesting push to what we might consider in our theory of stellar evolution. But science progresses also when we carefully check on each other's work, and that's what happened in this case."

Originally published on Live Science.

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