First ‘sleeping’ black hole discovered outside our galaxy

This black hole of a new type was revealed after six years of observation with the Very Large Telescope (VLT) from the European Southern Observatory (ESO) in Chile.

The bestiarium of the black hole grows with the first detection of a dormant black hole with stellar mass orbiting another star, still far enough away not to swallow its companion.

This new type of black hole, long predicted by theory but very difficult to detect as it is well hidden, was revealed after six years of observation with the Very Large Telescope (VLT) from the European Southern Observatory (ESO) in Chile, according to a study published Monday in Nature astronomy.

“We found a needle in a haystack,” lead author Tomer Shenar said in a statement. For three years, several candidates for the title of “sleeping black hole” had presented themselves, but so far no one had been accepted by this international team of astronomers, dubbed by ESO “black hole police”.

A massive star at the end of his life

The lucky winner, a dozen times the mass of the Sun, lurks in Magellanic Cloud, a dwarf galaxy near the Milky Way. It is like the other leg in a binary system of two stars turning around, one of which, dead, has turned into a black hole and the other is still alive.

Star mass black holes – incomparably smaller than their supermassive big brothers – are massive stars (between 5 and 50 times the mass of the Sun) at the end of their lives that collapse into themselves.

These objects are so dense and their gravity so powerful that not even light can escape: they are therefore by definition invisible. Scientists can nevertheless observe the substance circulating around before it is swallowed there … except when the black hole “sleeps”, on a diet.

25 times the mass of the Sun.

In the binary systems already observed, the star that has turned into a black hole is close enough to its companion star to “steal” its substance from it (we are talking about “growth”), explains Hugues Sana from the University of Louvain (KU Leuven ). ), in Belgium, one of the authors of the study.

Once captured, this material emits detectable X-rays. But here the black hole emits no one, and with good reason: “The living star (about 25 times the mass of the Sun) is far enough away not to be eaten. It currently remains in equilibrium on this orbit”, which lasts 14 days , the astronomer continues.

A balance that can not hold, according to him. “The living star will grow, and at this point part of its surface will be engulfed by the black hole”, which will then emit X-rays and therefore come out of its dormant state.

But how do you know such an object exists? “Imagine a pair of dancers holding hands as you observe in the dark. One has a black costume, the other a luminous costume: you only see the dance of the other, but you know he has a dance partner, thanks to the study of movement “, explains Hugues Sana.

Only a dozen black holes have been discovered so far

In astronomy, just as Jupiter and the Sun orbit each other, we can measure the respective masses of a binary system by observing these movements.

To make sure that the phantom object was actually a black hole, the researchers continued to eliminate and exclude several scenarios, such as a star losing its envelope.

“The only plausible explanation is that it is a black hole because no other star can reproduce this observational data,” the researcher sums up.

According to newer models, about 2% of the massive stars in our galaxy are likely to have a black hole around them, or about 100 million, according to Hugues Sana. “Currently, we only know about ten of them, all detected thanks to their x-rays, so we are missing some!”

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