Space: Discovering the largest dormant black hole

The matter contained in a black hole is so dense that nothing can escape its immense gravitational pull, not even light (hence the name black hole). The vast majority of stellar-mass black holes we know of so far engulf matter from a nearby stellar companion.

However, when a black hole has no companion close enough to steal matter from it, no light is produced and the black hole is extremely difficult to spot. These black holes are said to be "dormant".

A group of astronomers, including six from ULB's Astronomy and Astrophysics Institute, have just discovered one such object by analysing the unusual motion of its companion, an old giant star in the Aquila constellation, 1926 light-years away from Earth.

This dormant black hole, named Gaia BH3 ('Black Hole 3') – the third to be discovered thanks to Gaia – has the distinction of being the most massive known to date, at 33 times the mass of the Sun. Prior to this discovery, the mass record was held by a black hole in an X-ray binary in the Cygnus constellation (Cyg X-1), whose mass is estimated to be around 20 times that of the Sun.

ULB astronomers used the HERMES spectrograph mounted on the Mercator telescope of KU Leuven's Institute of Astronomy located at the Roque de los Muchachos Observatory (La Palma, Canary Islands) to confirm the orbital solution obtained from Gaia data for the binary system to which Gaia BH3 belongs. Their colleagues at the Royal Observatory of Belgium, the University of Liège and the University of Antwerp helped calibrate Gaia's high-resolution spectra, from which they obtained the precise velocity measurements needed to calculate the black hole's orbit around its stellar companion.

A great astronomical discovery establishing Belgium's expertise in international research!