It Turns out, the “Closest Black Hole” System Doesn’t Contain a Black Hole At All

One thousand light-years away is pretty close for a black hole. When researchers discovered a black hole at that distance in 2019, it caught the attention of other astronomers and other interested people. It was the first black hole-hosting stellar system to be seen with the naked eye.

But new research shows that it isn’t there.

The black hole got quite a bit of press when scientists published the paper presenting their discovery. It didn’t interact with the matter around it as other stellar-mass black holes do. Astronomers usually discover black holes as they draw material towards themselves, heat it, and release x-rays. But this one didn’t; it’s a non-accreting black hole, and astronomers discovered it through gravitational interactions with its two close neighbours. It was called a quiet black hole, and at the time, astronomers thought there must be many more of them and that the discovery was the tip of the iceberg.

“There must be hundreds of millions of black holes out there, but we know about only very few. Knowing what to look for should put us in a better position to find them,” said Thomas Rivinius, the lead author of the 2019 study.

But a new study by some of the same authors dug deeper into the hole and found that it’s not there. Something else better explains the data; Something called “stellar vampirism.” A more massive star draws matter away from a smaller companion in stellar vampirism, essentially feeding on a less massive donor star.

The system in question is called HR 6819. The 2019 paper announced a three-star system with a non-accreting black hole and two stellar companions. Researchers were looking for double-star systems when they found HR 6819. The closest star’s orbit was affected by the hole’s presence, they thought at the time. They learned the black hole’s mass by figuring out the closest star’s orbit. “An invisible object with a mass at least four times that of the Sun can only be a black hole,” concluded lead author Rivinius at the time.

The 2020 study was based on spectroscopy of the system with different instruments in different studies over a few years. Data from the ESA’s Gaia mission also played a role. The lack of x-rays from the black hole was an issue, but the 2019 study said the lack of x-rays was expected based on the system’s architecture.

“HR 6819 is a hierarchical triple star with a non-accreting BH in the inner binary,” the authors said in the conclusion of their 2019 paper. But something else they wrote foreshadows the new study showing that there is no black hole: “The detection was facilitated by the incompatibility of the spectral sequences with a simple binary.” It turns out they were correct when they said the spectral sequences were incompatible with a simple binary, but their conclusion was incorrect.

HR 6819’s spectral absorption lines are at the heart of the matter. HR 6819’s spectrum contained many different absorption lines: bright and dark, broad and narrow, moving and static.

The broad lines come from a Be star, a type of star behaving quite strangely. They’re giant blue stars that are hotter and more massive than the Sun. “Be stars actually rotate so rapidly that they almost fall apart,” Rivinius explained. “They are no longer spherical but somewhat flattened, and around their equator, they form a gaseous disc ejected from the star itself.” This hot gas around the star creates bright emission lines, clearly visible in the spectrum of HR 6819.

But there was a second set of spectral lines that were much narrower and more indicative of a different type of star called a B star. B stars are also massive blue stars, but they don’t spin as rapidly as Be stars. There was something odd about the lines from this B star: they moved back and forth every 40 days.

And this is where the black hole conclusion enters the story. For those lines to move back and forth like this, the B star had to be orbiting something. The unusual spectral sequences they observed in HR 6819 were incompatible with a simple binary. They couldn’t see a third star in the system, so the team concluded that there was a black hole at least four times more massive than the Sun was in the system.

There’s a third study involved in this narrative. Julia Bodensteiner, then a Ph.D. student at KU Leuven, Belgium, led that study published in 2020. In that study, the authors wrote that “We re-investigate the properties of HR 6819 to search for a possibly simpler alternative explanation for HR 6819, which does not invoke the presence of a triple system with a BH in the inner binary.”

“Our analysis suggests that HR 6819 is a binary system containing a stripped star and a Be star…” they wrote in their conclusion. “This work shows that the HR 6819 system can be explained without invoking the presence of a stellar-mass BH.” Other follow-up studies on HR 6819 also questioned the existence of the black hole.

When the studies doubting the existence of a black hole came out, it made Rivinius a little nervous. Could he and his team have gotten it wrong? “There were several papers,” remembers Rivinius, “and for some of them, I thought they proposed very viable hypotheses. When I realized that the no-black-hole scenario was indeed a viable hypothesis, at least as likely as ours, I started to sweat a little.”

Bodensteiner’s study had the same spectral lines as Rivinius’s, but they reached different conclusions. “Basically, the difference in our analysis is which of the spectral lines we used to study the motion of the Be star,” explained Bodensteiner. “Depending on which lines you use, you either get a significant result showing the Be star is moving back and forth or not.”

That’s where things stood until this new study was published. “We had reached the limit of the existing data, so we had to turn to a different observational strategy to decide between the two scenarios proposed by the two teams,” says KU Leuven researcher Abigail Frost, who led the new study published today in Astronomy & Astrophysics.

Researchers needed new data to solve the mystery, and they knew what facility could gather it. They knew there were two light sources in the system; the question was how closely they orbit each other. The two teams worked together to obtain new, sharper data of HR 6819 using ESO’s Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI). “The VLTI was the only facility that would give us the decisive data we needed to distinguish between the two explanations,” says Dietrich Baade, author on both the original HR 6819 study and the new Astronomy & Astrophysics paper. Since it made no sense to ask for the same observation twice, the two teams joined forces, which allowed them to pool their resources and knowledge to find the true nature of this system.

“The scenarios we were looking for were rather clear, very different and easily distinguishable with the right instrument,” says Rivinius. “We agreed that there were two sources of light in the system, so the question was whether they orbit each other closely, as in the stripped-star scenario, or are far apart from each other, as in the black hole scenario.”

“MUSE confirmed that there was no bright companion in a wider orbit, while GRAVITY’s high spatial resolution was able to resolve two bright sources separated by only one-third of the distance between the Earth and the Sun,” says Frost. “These data proved to be the final piece of the puzzle and allowed us to conclude that HR 6819 is a binary system with no black hole.”

While discovering a black hole only 1,000 light-years away is now…