A dormant black hole detected in the Large Magellanic Cloud
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It is the first "sleeping" stellar mass black hole detected in another galaxy.
A black hole is by definition a difficult star to observe since it emits no light. Nevertheless, many of them are caught in the middle of a meal, swallowing the matter that surrounds them, but also dust, gas or even stars for the largest. Before being ingested, these elements enter into a crazy race around the black hole and, in doing so, emit large quantities of radiation, especially in X-rays. This is how astrophysicists can spot them. Some black holes are less voracious and emit almost no electromagnetic waves. They can thus escape the instruments that scan the sky. It is such an object that has just been discovered in the Large Magellanic Cloud, one of the satellite galaxies of the Milky Way.
Around a warm and blue star
A team of international experts, renowned for having debunked several black hole discoveries, discovered this stellar mass black hole in the VFTS 243 system. Weighing 9 solar masses, it orbits a hot blue star 25 times the mass of the Sun. Its identification is not yet completely certain, but as the authors of the study published in the journal Nature Astronomy point out, it is so far the strongest candidate.
To find it, the team studied the Tarantula Nebula, a vast star-forming region in the Large Magellanic Cloud, for several years using the FLAMES instrument, a spectrograph on ESO's Very Large Telescope. The collaboration scanned nearly 1,000 massive stars in search of those that might have black holes as companions. It is extremely difficult to identify these companions as black holes because there are many other possibilities: neutron stars, brown dwarfs, giant planets... Finally, it is VFTS 243 which seems to be the most promising.
A missing star
Stellar mass black holes form when massive stars reach the end of their lives and collapse under their own gravity. In a binary, a system of two stars revolving around each other, this process leaves behind a black hole orbiting a bright companion star. These usually have an average mass between 5 and 10 solar masses. However, the gravitational wave detectors LIGO and VIRGO have indirectly observed black holes of about 30 solar masses. And at the other extreme, the Chandra telescope has revealed a black hole of barely 2.7 solar masses, resulting from the collision of two neutron stars.
Another peculiarity of this black hole: it was formed without the star of which it is the remnant, has exploded as a supernova. "The star that formed the VFTS 243 black hole appears to have collapsed entirely, with no sign of a previous explosion," explains Tomer Shenar of the University of Amsterdam in an ESO release. "Evidence for this direct collapse scenario has emerged recently, but our study provides perhaps one of the most direct indications. This has huge implications for the origin of black hole mergers in the cosmos," he concluded.