Space. Discovery of a new black hole, the very first “sleeper”

The black hole bestiary grows with the first detection of a dormant stellar-mass black hole orbiting another star still far enough away not to swallow its companion. This binary system responds to the cute name of VFTS 243.

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

“The Black Hole Police”, an international team

“We found a needle in a haystack,” Tomer Shenar, its lead author, said in a statement. For three years, several candidates for the title of “sleeping black hole” had presented themselves, but none had so far been accepted by this international team of astronomers, dubbed by ESO the “black hole police”.

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

Stellar 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, which collapse in on themselves.

A “sleeping” black hole, hard to perceive

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

In the already observed binary systems, the star that has become a black hole is close enough to its companion star to “steal” its matter from it (we speak of “accretion”), explains Hugues Sana from the University of Louvain (KU Leuven ). ), in Belgium, one of the authors of the study.

When this material is trapped, it emits x-rays which can be detected. But here the black hole emits none, 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 cannot 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 emerge from its dormant state.

The study of movement makes it possible to discover the invisible

But how do you know such an object exists? “Imagine a couple of dancers holding hands and seeing them in the dark. One has a black suit, the other a light suit: you only see the dance of the other, yet you know that he has a dance partner , thanks to the study of movement”, explains Hugues Sana.

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

Only about ten known black holes… out of 100 million

To be sure that the phantom object was indeed a black hole, the researchers continued to eliminate and rule out several scenarios, such as a star losing its envelope. “The only reasonable explanation is that it is a black hole, because no other star can reproduce these observational data”, summarizes the researcher.

According to recent 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. “At the moment we only know about ten of them, all recorded thanks to their X-ray emission, so we are missing some! »

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