Apparently, that "monster black hole" that researchers found isn't so monstrous after all. But finding errors and working to correct them in how science pushes forward.
In a recent study (a peer-reviewed study published Nov. 27), a team of scientists reported the discovery of the binary system LB-1, which contains a star and, according to the findings, a black hole companion 70 times the mass of our sun. This was major news, a stellar-mass black holes (black holes formed by the gravitational collapse of a star) are typically less than half that massive. But while the study, led by Jifeng Liu, of the National Astronomical Observatory of China (NAOC) of the Chinese Academy of Sciences, was exciting, it was also wrong.
Three new papers came out this week that reexamined the findings from Liu's study, and these studies say that LB-1's black hole isn't actually all that massive.
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Strange black holes
Stellar-mass black holes are typically identified by the bright X-ray emissions that come from the gas that the objects accrete, or pull in, from their companion stars. But the black hole spotted in LB-1 is "noninteracting"; in other words, it doesn't accrete gas from its star, so it can't be found through bright emissions. Scientists think that there are many examples of this type of black hole in the universe, but because these objects are tough to spot, there are few observations to show just how many might be out there.
So, to determine that the system had a black hole, Liu's team had to find and study the object indirectly, by observing movement in the Doppler shift of the system's star and a deep-red emission line.
Under the Doppler phenomenon, objects moving toward Earth appear blue, because the light wavelengths are getting shorter, and red when moving away from us, because the wavelengths are getting longer. The emission line, known as an H-alpha emission line, is a spectral line, or a dark line in a spectrum. Spectral lines are often used to identify atoms or molecules and this specific line is created by hydrogen electrons. Liu's team completed their work under the presumption that this line was coming from the accretion disk around the black hole.
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By measuring changes in the Doppler shift, researchers could determine the velocity of the objects and, therefore, their mass. "If the star and the companion were accelerating the same amount, that would mean they have the same mass, and if one is accelerating much less, it would be much heavier," University of California, Berkeley, astronomy doctoral student Kareem El-Badry, a co-author on one of the three papers analyzing these results, said. So, in measuring the wiggling movement of the emission coming from (what Liu's team assumed was) the black hole, Liu's team determined that the velocity of the black hole must mean it was extremely massive for a stellar-mass black hole.
Now, if the emission was, in fact, coming from a black hole and moving as they reported, that indeed would mean that there was an extremely massive object in the system, El-Badry explained.
The main problem with this conclusion? It turns out that that this emission line, the motion of which served as the main evidence for the proposed ultramassive object, wasn't wiggling. In fact, it wasn't moving at all, the new papers addressing Liu's team's conclusions found.
A bold claim
You may have heard talk in the last couple weeks about an “impossible” 70 solar mass black hole. In today's dose of cold water, we argue that the data was misinterpreted, and there is no evidence of an unusually massive BH. 1/ https://t.co/hWLhvaFK1F pic.twitter.com/FoEPifPegcDecember 10, 2019
The claim of a strangely massive black hole discovery first struck El-Badry as strange, because this type of black hole has never before been observed with such a mass. "My first thought when the paper came out is this is such a bold claim that the evidence better be really good," El-Badry told Space.com. "You should always keep an open mind, but in this case, the claim was definitely extraordinary and the evidence was a bit more shaky."
The main issue that El-Badry found was that the emission line only appeared to be moving; it wasn't actually wiggling.
El-Badry and Eliot Quataert, a professor of astronomy and physics at UC Berkeley, published their analysis on Monday (Dec. 9) to the preprint server arXiv. Their paper has also been submitted for publication in the journal the Monthly Notices of the Royal Astronomical Society.
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A missing absorption line
So, how can an emission line only "appear to be moving"? Well, it just so happened to line up on top of an absorption line, which created the illusion.
To understand the illusion, you first need to know what an absorption line is. The outer atmospheric layers surrounding stars serve as an absorbing material to absorb light coming from the star. So, when researchers study the spectrum of light coming from stars, they can see absorption lines, which are created by atoms in the atmosphere transitioning between atomic states.
With the star in LB-1, there was an absorption line "hidden" by the emission line, El-Badry said. Such a situation can create the illusion that the emission line is moving, producing the appearance of Doppler shift, which El-Badry and the scientists behind the other papers explained and showed in the studies. By simply subtracting the absorption line from measurements of the emission line, El-Badry and Quataert, who used the same data for their study as Liu's team did, found that the emission line wasn't moving at all.
Without the movement of this emission, Todd Thompson, a professor in the Department of Astronomy at The Ohio State University, who wasn't involved in any of these papers, explained to Space.com, there are two possible interpretations. Either the second object in the system is far more massive than has ever been observed (way more than 70 solar masses) or, much more likely, there could just an average-size black hole in LB-1 and the emission line is coming from somewhere else, Thompson said.
"There's something there. It's just that it's probably just a regular, stellar-mass black hole," Jackie Faherty, a senior scientist at the American Museum of Natural History in New York and a co-host of "StarTalk Radio," told Space.com. Faherty wasn't involved in any of these papers.
However, because the emission line is probably not coming from the black hole, researchers can't get a super precise estimate of the black hole's mass. But the analysis of El-Badry's team suggests that the black hole is most likely between 5 and 20 solar masses, which, as they described in their paper, "seems most plausible."
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Discovery ... busted?
Two additional papers have come out that also reexamine the claims made by Liu's team. One, a study led by New Zealand theoretical astronomer J.J. Eldridge, which has been published to arXiv, took a theoretical approach to analyzing the system. Researchers in that study simulated a large library of different kinds of binary systems to see if the scientists could find a binary that matched the observations reported for LB-1. They found several that could, but none with 70-solar-mass black holes.
The other study, also published to arXiv, and led by Michael Abdul-Masih of the Institute of Astronomy at the university KU Leuven in Belgium, took a similar approach to El-Badry's. However, instead of using the same data as Liu's team, these researchers collected their own spectrum of the binary system using a different telescope. They also did simulations in which they put an absorption line beneath an emission line to see if the emission appeared to move as the one in LB-1 did. In these simulations, Abdul-Masih's team found that the line did appear to move back and forth, providing further evidence that the emission line in the system only looks like it's moving.
Redemption for LB-1
"It did seem a little too exciting to be true," Faherty said. But, she added, "this is also the way science progresses."
Faherty emphasized that "This is OK for this kind of thing to happen. … It's just a correction to a previous result … it's OK to have this kind of situation," she added. "Science advances and moves forward."
These follow-up studies have provided evidence that the secondary object in LB-1 is not actually an ultrarare, ultramassive black hole. However, it is still an exceptionally interesting object and worth studying further, El-Badry said.
Because there has been so much attention on the original study, including with these follow-up analyses, it has increased interest in the study of the LB-1 system and systems like it.
By identifying and studying noninteracting black holes like the one associated with LB-1, scientists can learn more about these elusive objects. Said to be common in space, they are tough to spot, because they don't produce bright X-ray emissions.
"It's a very interesting time to go looking for these noninteracting black holes, and they definitely have found a very interesting system," Thompson said. There is a "population that must be out there of black holes in stellar binaries where there's no active interaction between the two components," he added.
Additionally, it could be interesting if scientists continue to investigate where exactly this H-alpha emission line is coming from. The papers reexamining LB-1 suggest "that it's possible that circumbinary material could account for it, but it's a slight mystery … it's OK to have some mystery involved in a result," Faherty said.
Space.com reached out to Liu's team for comment, and Liu said that "We are writing a paper to address all these concerns." He added that his team expects that paper to be out sometime next week.
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