Scientists made waves last year after announcing the discovery of a "monster black hole," unlike anything seen before. But, as researchers continue to weigh in, one thing is clear: there is no "monster."
Solar mass black holes, or black holes that form when a star collapses under the influence of its own gravity, are usually about 20-30 times the mass of our sun. So it was breaking news when, in November 2019, scientists led Jifeng Liu of the National Astronomical Observatory of China (NAOC) of the Chinese Academy of Sciences, reported the discovery of a black hole 70 times as massive as our sun.
This would have been the most massive black hole ever discovered (by far) — a groundbreaking find that would have drastically changed our perception of these objects. However, following this work, a number of scientists questioned these findings as they seemed improbable. In fact, there were a handful of studies that found evidence that completely disproved the possibility of such a black hole.
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In December 2019, Kareem El-Badry, an astronomy doctoral student at the University of California, Berkeley, co-wrote a paper published online in the preprint server arXiv providing evidence that this wasn't a 70 solar mass black hole. (The paper was later published in the Monthly Notices of the Royal Astronomical Society.)
Around the same time, two additional papers disproving the discovery were published on arXiv: one from a team led by J.J. Eldridge, a theoretical astronomer at the University of Auckland in New Zealand, and another led by Michael Abdul-Masih, a PhD student from the KU Leuven Institute of Astronomy in Belgium.
On Wednesday (April 29), Abdul Masih's paper was published in the journal Nature. In this paper, the team analyzed the system in depth and concluded that there is "no evidence for a massive black hole," they wrote in an accompanying commentary.
That same day, Liu's team responded with their own Nature commentary in which, while they disagree with some points made by other researchers, they do agree that there is no 70 solar mass black hole in the system.
The original "discovery"
Liu's team originally found the object in question when they discovered the binary, or two-object system LS V +22 25 (or LB-1 for short), which they described in a peer-reviewed study published Nov. 27, 2019 in Nature. They described a system with a 70 solar mass black hole and an 8 solar mass star orbiting each other. The star, because it was bright and obvious, was easy to spot. But the alleged monster black hole? Not so much.
Usually, in systems with stellar-mass black holes, there is a bright, X-ray emission shooting out of the system that scientists can use to identify it. This emission line is created when a black hole accretes, or pulls material from the other object (in this case a star) in the system. But since LB-1's black hole doesn't accrete material from its partner star, it doesn't create an X-ray emission line, Liu's team found. This made it a little trickier to study.
So, to identify the second object in the system the team had to rely on a more subtle signature known as an H-alpha emission line. This is a spectral line, or a dark line in an object's observed light spectrum that can be used to identify which molecules or atoms make up the material it's coming from.
Liu's team presumed that this H-alpha emission line was coming from an accretion disk, or disk of gas and dust that the black hole pulls in from other objects around the black hole. By observing how this emission line seemed to wobble, they determined the orbital motion and the size of the black hole.
But the interpretation of this subtle wobbling signature, this H-alpha emission line which led Liu's team to determine the existence and massive size of a black hole, is the main finding other researchers have a problem with.
No "monster black hole"
The three papers published initially to the preprint server arXiv in 2019 in response to Liu's alleged discovery found that the movement of this faint H-alpha emission line was actually a kind of optical illusion, and that the orbital motion Liu's team calculated to determine the black hole's size was inaccurate.
"Instead of the wobble coming from the black hole, they found it was more likely a consequence of the orbital motion of the subgiant [its star], thus completely reducing the mass of the black hole," Jackie Faherty, a senior scientist at the American Museum of Natural History in New York and a co-host of "StarTalk Radio," who wasn't involved in any of these papers, told Space.com in an email.
In their paper, Abdul-Masih's team used observations they have made using the Flemish Mercator telescope at La Palma in the Canary Islands over the last three months (or since their last paper on this subject) and have "found that that the signal that they use to measure the mass of the black-hole was actually fake," co-author and Hugues Sana, a research associate professor at KU Leuven, told Space.com in an email. Sana added that, in this paper, Abdul-Masih "showed that the same apparent signal is produced by the motion of the spectral line of the visible B-star companion."
So, Sana explained, "once we take into account the motion of the B-type star spectrum, the signal used to calculate the high mass of the (putative) black hole disappears and no indication of a high-mass black hole is left in the data."
To summarize their main finding, "We show that this complicated emission line is contaminated by an absorption line associated to the star, which Liu and his team did not account for," Abdul-Masih told Space.com in an email.
But Abdul-Masih's team didn't just find more evidence to prove that this object isn't a 70 solar mass black hole. They also have a better idea of what may actually be in the LB1 system.
"By analyzing these new high-resolution data, we have come to the conclusion that the LB1 binary system is likely formed by a stripped star and a more massive rapidly rotating Be star," Sana said. (Be stars are types of stars with B-type spectral types and emission lines.) "The strong H-alpha emission is coming from the Be star," Sana said. "The Be star signature was not seen in the original data because the spectral lines are very broad, very shallow, and do not move much (because the Be star is more massive than the stripped B-star)."
"Abdul-Masih's team obtained new high-spectral resolution data which helps with the analysis. Reading the paper, it seems like a sound and robust analysis," Faherty added.
Sana concluded that, while this means that LB1 looks a lot different than Liu's team originally suggested, it's still an extremely interesting system. "Such a stripped star+Be star system is very rare, so we are still very excited about what can be learned from LB1," they said.
An agreement and rebuttal
Liu's team was quick to respond to Abdul-Masih's new paper, with their own commentary published in Nature on the same day. In their commentary, which is a direct response to Abdul-Masih's team, Liu and his team "re-examine those arguments, in light of more recent spectroscopic observations and further analysis," they wrote in this piece. However, while Liu's team doesn't agree with all aspects of the new analysis of the system, they do agree on a number of points.
They agree that the possibility of a 70 solar mass black hole in LB1 can be completely ruled out. "The response does not disagree with the possibility of the interpretation by the previously noted authors," Faherty said.
"I am very glad that Liu and his team are receptive to our interpretation of the system. This system is indeed very complex and I think that their analysis of the new near-infrared data could be very informative," Abdul-Masih said.
However, unlike Abdul-Masih and the other researchers who have responded to the original paper, Liu's team still asserts that it's possible that the black hole in this system is between 23 and 65 solar masses. "But that comes with all their caveats," Faherty said.
"The most telling statement that they make in the letter is 'We accept that the interpretation of the H-alpha profile is more complex than originally envisaged.'" Faherty said.
She added that while the system is likely what El-Badry and Abdul-Masih's teams described, "as Liu and co-authors state, the jury is still out. More detailed observations are necessary to pin down what this system might include," she said.
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