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Discovery of the two black holes closest to Earth and they are of a type never seen before!

A star oscillating in the sky is not entirely normal. At least, it hides the presence of another massive object in the vicinity. And when no instrument is able to detect the light, there is only one option left: it is a black hole. More exactly of the first two black holes of a whole new family. Found in the outskirts of Earth, moreover.

Gaia is what scientists call an astrometric mission. A European Space Agency (ESA) mission dedicated to measuring the position, distance and movement of stars. And it is precisely in this way, by measuring the movements of a few stars, that Gaia has just put astronomers on the trail of the two closest black holes to Earth ever discovered.

Often, black holes are betrayed by the light – usually X-rays or radio waves – emitted by the matter they engulf. This time it’s strange “wobbles” of stars that revealed the hidden presence of two very massive objects. Of each about 10 times the mass of our Sun. One, dubbed Gaia BH1, some 1,560 light-years from our Earth, in the direction of the constellation Ophiuchus. The other, baptized Gaia BH2, about 3,800 light-years from us, in the direction of the Centaur.

“What distinguishes this new group of black holes from those we already knew is their wide separation from their companion stars. – probably a red giant star as far as BH2 is concerned. These black holes probably have a completely different formation history than the X-ray binaries.”explains Kareem El-Badry, discoverer of these new black holes and researcher at Harvard-Smithsonian Center for Astrophysics (United States) and Max-Planck Institute for Astronomy (Germany), in an ESA press release. ” X-ray binaries » being black holes whose companion stars are close enough to them that the system shines in X-rays and at radio wavelengths.

Still lots of black holes in Gaia’s data?

To arrive at this discovery, it will have been necessary to mobilize all the power and the unequaled precision of Gaia. Because it is a question here of tiny oscillations detected in the movement of stars. And once the chip was put in the ear of astronomers, measurements made by ground observatories confirmed that the mission had indeed detected new black holes.

The astronomers then wanted to look on the side of Gaia BH2, the radio or X emissions that are familiar to them when it comes to observing a black hole. With the support of two powerful instruments, too: the Chandra X-ray observatory (Nasa) and the South African radio telescope MeerKAT on the ground. But nothing. “This tells us that the black hole is not a big eater. – of those that scientists call “sleeping black holes”and that few particles cross its event horizon. We don’t know why, but we want to know.”says Yvette Cendes, astronomer atHarvard-Smithsonian Center for Astrophysics.

Especially since researchers now expect to discover that there are many black holes of this new kind. While waiting to be able to look for them in the new series of Gaia data which will be published in 2025 – quite a few candidates could be flushed out there – astronomers will work to adapt their theories to these brand new observations.

Written by Emilie Grenaud

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