The list of space phenomena grew 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 as it is well hidden, has revealed itself after six years of observation with the European Southern Observatory’s (ESO) Very Large Telescope (VLT). in English) in Chile, according to a study published in the scientific journal Nature Astronomy on Monday (18).
“We found a needle in a haystack,” commented the study’s lead author, Tomer Shenar, in a statement.
For three years, several candidates for the title of “sleeping black hole” have been put forward, but none have so far been accepted by this international team of astronomers, dubbed by ESO the “black hole police”.
The winner, with a dozen times the mass of the Sun, is in the Magellanic Cloud, a dwarf galaxy close to the Milky Way. It’s like the second leg of a binary system of two stars revolving around each other, one of which, dead, has become a black hole and the other is still alive.
Stellar-mass black holes — incomparably smaller than their supermassive brethren — are massive stars (between 5 and 50 times the mass of the Sun) at the end of their lives, which are collapsing in on themselves.
These objects are so dense and their force of gravity so powerful that even light cannot escape: they are therefore, by definition, invisible. Scientists can, however, observe the matter that circulates around it, before it is swallowed… Except when the black hole “sleeps”, on a diet.
In binary systems already observed, the star that became a black hole is close enough to its companion star to “steal” its matter (“accretion”), explains to AFP Hugues Sana, from the University of Louvain (KU Leuven), in Belgium, one of the authors of the study.
‘Dancer couple’
This matter, once captured, emits X-rays, which can be detected. But here, the black hole emits none, and for good reason: “The living star (about 25 times the mass of the Sun) is far enough away not to be swallowed. It remains, for now, in equilibrium in this orbit.” , lasting 14 days, continues the astronomer.
A balance that will not last, according to him. “The living star will grow, and at this point part of its surface will be swallowed up by the black hole,” which will emit X-rays and therefore come out of its dormant state.
But how do you know that such an object exists? “Imagine a couple of dancers holding hands, which you watch in the dark. One black dress, the other a luminous costume: you only see the second dance, but you know he has a dance partner, thanks to the study of movement” , explains Hugues Sana.
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 indeed 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 this observational data”, summarizes the researcher.
According to recent models, about 2% of the massive stars in our galaxy likely have a black hole around them, or about 100 million, according to Hugues Sana.
“At the moment, we know of only ten of them, all detected thanks to their X-ray emissions, so we are missing some!”
