Discovery of a ‘passive’ stellar mass black hole

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

This new type of black hole, long predicted by theory but so difficult to detect because it is so well hidden, was revealed after six years of observations by the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile. The study was published Monday in the journal Nature Astronomy.

“We found a needle in a haystack,” lead author Tomer Schöner said in a statement. For three years, many candidates for the title of “sleeping black hole” have presented themselves, but so far none has been accepted by this international group of astronomers, baptized by the ESO “police of black holes”.

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 orbiting two stars, one of which has died and turned into a black hole, while the other is still alive.

Stellar-mass black holes – incomparably smaller than their supermassive brothers – massive stars (5 to 50 times the mass of the Sun) at the end of their lives, which collapse in on themselves.

These objects are so dense and their gravity so powerful that even light cannot escape: they are invisible by definition. However, except when a black hole is “sleeping,” scientists can observe matter circulating before it is swallowed by food.

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

“Dancing Couple”

This material, once captured, emits X-rays that can be detected. But here, the black hole emits nothing, and for good reason: “It’s too far away to eat a living star (about 25 times the mass of the Sun). It is balanced in this orbit”, which lasts 14 days, continues the astronomer.

An unsustainable balance for him. “A living star grows, and during this time, part of its surface is swallowed up by a black hole,” which emits X-rays and therefore comes out of its dormant state.

But how do you know such an object exists? “Imagine two dancers you observe in the dark holding hands. One has a black dress, the other a bright dress: you only see the second dance, but you know he has a dance partner, thanks to the study of movement”, explains Hughes Sana.

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

To confirm that the phantom object was indeed a black hole, the researchers proceeded by elimination, ruling 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,” the researcher summarizes.

According to the latest models, about 2% of the massive stars in our galaxy have black holes orbiting them, or about 100 million, according to Hughes Chana. “At the moment, we only know about ten of them, and they’ve all been detected by X-ray emissions, so we’re missing some!”