'Sleeping' stellar-mass black hole found, very rare object - rts.ch

The black hole bestiary grows with the first detection of a dormant stellar-mass black hole. It revolves around another star, still far enough away not to swallow its companion. Astronomers call it “a needle in a cosmic haystack”, given its rarity.

Some 160,000 light years away is the Tarantula Nebula, the most spectacular structure in the Large Magellanic Cloud. This region reveals a celestial landscape dotted with star clusters, glowing gas clouds and scattered remnants of supernovae explosions. [ESO]This new type of black hole – predicted for a long time by theory but very difficult to detect as it is so well hidden – was revealed at the end of six years observation with the instrument FLAMES on the Very Large Telescope (VLT) of the European Southern Observatory (IT’S) in Chile, according to a study appeared Monday in Nature Astronomy.

“We found a needle in a cosmic haystack” (read first box), welcomed Tomer Shenar, its main author, in a press release. 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, baptized by the ESO the “black hole police”. “This has huge implications for the origin of black hole mergers in the Cosmos,” says the researcher.

The lucky winner, a dozen times the mass of the Sun, is lurking in the Magellanic Cloud, a dwarf galaxy near the Milky Way. It is like the second leg of a binary system of two stars turning around, one of which, dead, has become a black hole and the other is still alive: it is an extremely luminous blue star and hot.

>> Watch the VFTS binary system animation

The system is composed of a hot, blue star twenty-five times the mass of the Sun and a black hole at least nine times the mass of the Sun. Sizes are not to scale: in reality, the blue star is about 200,000 times bigger than the black hole:

Trou noir VFTS 243 eso [RTS] — Star and black hole VFTS 243 (video: ESO/L. Calçada) / Video news / 17 sec. / today at 12:16

To find VFTS 243, the collaboration searched nearly 1,000 massive stars in the Tarantula Nebula region of the Large Magellanic Cloud, looking for any that might have black holes as companions. It is extremely difficult to identify these as black holes, as there are many other possibilities.

A collapsed star

Stellar-mass black holes – incomparably smaller than their supermassive big 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 (read second box).

These objects are so dense and their force of gravity so powerful that not even light can escape: they are therefore by definition invisible. Scientists can nevertheless observe the matter which circulates around, before being swallowed there… except when the black hole “sleeps”, on a diet.

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

This material, once caught, emits X-rays, which can be detected. But here, the black hole does not emit any, and for good reason: “The living star – about 25 times the mass of the Sun – is far enough away not to be eaten. It remains for the moment in equilibrium on this orbit”, lasting fourteen days, continues the astronomer.

A balance that cannot last, according to him: “The living star will grow and, at this time, part of its surface will be swallowed up by the black hole”, which will then emit X-rays and therefore come out of its dormant state. .

Watch the dance moves

But how do you know that such an object exists? “Imagine a dancing couple holding hands, whom you observe in the dark. One has a black suit, the other a luminous suit: you only see the dance of the second, 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 revolve around each other, we can measure the respective masses of a binary system by observing these movements.

To be sure that the phantom object was indeed a black hole, the research team proceeded by elimination, ruling 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 these observational data,’ sums up the researcher.

>> Journey from the Milky Way to the Tarantula Nebula

Flight from our home galaxy to the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way, about 160,000 light years from Earth [ndlr. une année-lumière est la distance que la lumière parcourt en une année, soit 9,5 trillions de kilomètres]. It is home to one of the brightest known nebulae, the Tarantula Nebula, discovered in the mid-1800s.^e century. This hosts the VFTS 243 binary system, the final destination of this video: