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The closest black hole to Earth is not a black hole

What until now was 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 this Wednesday.

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, that would become the closest to our planet, but the results of their study were contested by several research institutions, including an international team based at the Catholic University of Leuven (Belgium).

In an article published this Wednesday in the journal Astronomy & Astrophysics, teams from ESO and the Belgian university come together to report that, in fact, there is no black hole in HR 6819, but instead it is a two-star system “vampire” in a rare stage and of short duration of its evolution.

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

“Not only is it normal, but it should be common that the results are reviewed. he says, “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 HR 6819 was a triple system, with a 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 could also be a system with just two stars in a 40-day orbit and no black holes 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 on Wednesday.

To solve the mystery, the two teams worked together to get new, sharper data from HR 6819, joining their resources and knowledge to solve the unknown about the true nature of this system.

“The scenarios we were looking for were quite clear, very different and easily distinguishable with the right instrument,” says Rivinius.

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

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 receiving star began to spin faster,” says the researcher.

“Capturing such a post-interaction phase is extremely difficult because it’s so short,” adds Frost. “This makes our findings on HR 6819 are very exciting, since 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”.

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