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 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 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 been presented, but none had been accepted so far by this international team of astronomers, baptized by ESO as “black hole police”.
A massive star at the end of its life.
The lucky winner, a dozen times the mass of the Sun, lurks in the Magellanic Cloud, a dwarf galaxy near the Milky Way. It is like the second leg of a binary system of two rotating stars, one of which, dead, has become 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, collapsing in on themselves.
These objects are so dense and their gravity so powerful that not even light can escape: therefore, they are by definition invisible. However, scientists can observe the matter circulating, before it is swallowed there… except when the black hole “sleeps”, on a diet.
25 times the mass of the Sun
In binary systems already observed, the star that has become a black hole is close enough to its companion star to “steal” its matter (we speak of “accretion”), explains Hugues Sana of the University of Leuven (KU Levin). , in Belgium, one of the authors of the study.
This material, once trapped, emits X-rays, which can be detected. But here, the black hole doesn’t emit anything, and for good reason: “The living star (about 25 times the mass of the Sun) is far enough away that it won’t be devoured. It remains in equilibrium in this orbit for the time being.” , lasting 14 days, continues the astronomer.
A balance that cannot last, according to him. “The living star will grow and at that time part of its surface will be swallowed by the black hole,” which will then emit X-rays and thus come out of its dormant state.
But how do you know that such an object exists? “Imagine a couple of dancers holding hands, whom you observe in the dark. One has a black suit, the other a light suit: you only see the dance of the second, but you know that he has a dance partner, thanks to the study of movement”, explains Hugues Sana.
Only a dozen black holes detected so far
In astronomy, just as Jupiter and the Sun revolve around each other, we can measure the respective masses of a binary system by observing these motions.
To be sure that the phantom object was in fact a black hole, the researchers proceeded by elimination, ruling out various 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.
Based on 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 detected thanks to their X-ray emissions, so we are missing a few!”
Source: BFM TV
