The closest black hole to Earth is not: what happened to this space object?

What was previously believed to be the closest black hole to Earth, just a thousand light-years away, has turned out not to be, according to a new study published today in the journal Astronomy & Astrophysics.

A team led by astronomers from the European Southern Observatory (ESO) reported in 2020 the discovery of a black hole in the HR 6819 system, which would become , but the results of their study were contested by several research institutions, including found an international team based at the Catholic University of Leuven (Belgium).

In the paper, the ESO and Belgian university teams team up to report that there is in fact no black hole in HR 6819, but rather a rare and short-lived stage in its evolution.

The original study on HR 6819 received a lot of attention from both the press and the scientific community. Thomas Rivinius, an ESO astronomer based in Chile and lead author of that paper, was not surprised by the reception by the astronomical community to his discovery of the black hole.

“Not only is it normal, but it should be common for the results to be reviewed,” it states, “and a result that hits the headlines, even more so.”

Rivinius and his colleagues were convinced that the best explanation for the data they had was that , with one star orbiting a black hole every 40 days and a second star in a much wider orbit.

But a study led by Julia Bodensteiner, then a doctoral student at the Catholic University of Leuven, proposed a different explanation for the same data: HR 6819 too, and no black hole at all.

This alternative scenario would require one of the stars to be “stripped” of a large part of its mass, meaning that, at an earlier time, this mass had been “stolen” by another star.

”We had reached the limit of the existing data, so we had to resort to a different observation strategy to decide between the two scenarios proposed by the two teams.”, says the researcher at the Catholic University of Leuven, Abigail Frost, who led the new study published today.

To solve the mystery, the two teams worked together to obtain new and sharper data on HR 6819, pooling their resources and knowledge to shed light on the true nature of this system.

”The scenarios we were looking for were quite clear, very different and”, Rivinus says.

“We agreed that there were two light sources in the system, so the question was whether they orbited each other closely, as in the stripped stars scenario, or were they far apart from each other, as in the black hole scenario. ”, Explain.

The new findings of this joint team led to the conclusion that HR 6819 is a binary system without a black hole.

“Our best interpretation so far is that we picked up this binary system shortly after one of the stars had sucked the atmosphere from its companion star. This is a common phenomenon in close binary systems, sometimes referred to as stellar vampirism in the press.”explains Bodensteiner, now a member of ESO in Germany and an author of the new study.

“While the donor star was stripped of some of its material, the recipient star began ”says the researcher.

“Capturing a phase of this type, after the interaction, is extremely difficult, since it is very short”Frost adds. “This makes our findings on HR 6819 very exciting, as it is a perfect candidate to study how this vampirism affects the evolution of massive stars and, in turn, the formation of associated phenomena, including gravitational waves. and violent supernova explosions.”

The newly formed team, bringing together Leuven and ESO, plans to monitor HR 6819 more closely using the VLTI’s GRAVITY instrument. The team will carry out a joint study of the system over time to better understand its evolution, to learn more about other binary systems.

As for the search for black holes, the team remains optimistic. For Rivinus, “Stellar-mass black holes are very elusive due to their nature.” “But order-of-magnitude estimates –adds Baabe– suggest that there are tens to hundreds of millions of black holes in the Milky Way alone.” It is only a matter of time before the astronomical community discovers them.

Agencies