The black hole bestiary grows with the first discovery of a dormant stellar-mass black hole orbiting another star that is still far enough away not to swallow its companion.
Long predicted theoretically but very difficult to detect because they are well hidden, this new type of black hole was discovered after six years of observations with the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile. published in Nature Astronomy on Monday.
“We found a needle in a haystack,” Tomer Shenar, the lead author, said in a statement. For three years, several contenders for the title “Sleeping Black Hole” had presented themselves, but none had been accepted by this international team of astronomers, dubbed by ESO the “Black Hole Police”.
The lucky winner, a dozen solar masses, lurks in the Magellanic Cloud, a dwarf galaxy near the Milky Way. It is like the second leg of a binary system of two orbiting 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 that are collapsing in on themselves.
These objects are so dense and their gravity so strong that not even light can escape: they are therefore, by definition, invisible. Scientists can nonetheless observe the material circulating around it before it is swallowed…. except when the black hole is “snoozing”, on a diet.
In the binaries already observed, the star-turned-black-hole is close enough to its companion to ‘steal’ (we call it ‘accretion’) its matter, explains Hugues Sana of the University of Louvain to AFP. (KU Leuven), Belgium, one of the authors of the study.
Once captured, this material emits X-rays that can be detected. But here the black hole isn’t emitting, and for good reason: “The living star (about 25 times the size of the Sun) is far enough away not to be eaten. It remains in equilibrium in this orbit for the time being,” continues the astronomer, and lasts 14 days.
A balance that, in his opinion, cannot last. “The living star will grow, and at that point part of its surface will be swallowed up by the black hole,” which then emits X-rays and wakes up from its dormant state.
But how do you know that such an object exists? “Imagine a couple of dancers holding hands and watching them in the dark. One has a black suit, the other a light-colored suit: you only see the other person dancing, but thanks to the movement study, you know that he has a dance partner,” explains Hugues Sana.
In astronomy, just as Jupiter and the Sun rotate around each other, we can measure the respective masses of a binary star system by observing these movements.
To be sure that the phantom object was actually a black hole, the researchers used elimination and ruled out several scenarios, such as that of a star losing its envelope.
“The only reasonable explanation is that it is a black hole, because no other star can reproduce this observation data,” summarizes the researcher.
According to current models, around 2% of the massive stars in our galaxy are likely to have a black hole around them, or around 100 million, according to Hugues Sana. “At the moment we only know about ten of them, all of which were discovered thanks to their X-ray radiation, so we’re missing a few! »