All stellar-mass black holes have hitherto been identified by X-rays emitted from gas that is accreting onto the black hole from a companion star. These systems are all binaries with a black-hole mass that is less than 30 times that of the Sun. Theory predicts, however, that X-ray-emitting systems form a minority of the total population of star–black-hole binaries. When the black hole is not accreting gas, it can be found through radial-velocity measurements of the motion of the companion star. In a recent paper, "A wide star-black-hole binary system from radial-velocity measurements", published in Nature on Nov.28, 2019, an international team of astronomers, including Krzysztof Belczyński from Copernicus Center in Warsaw, reported radial-velocity measurements taken over two years of the Galactic B-type star, LB-1. They find that the motion of the B star and an accompanying Hα emission line require the presence of a dark companion with a mass of 68+11−13 solar masses, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. Gravitational-wave experiments have detected black holes of similar mass, but the formation of such massive ones in a high-metallicity environment would be extremely challenging within current stellar evolution theories.
Krzysztof Belczyński and his collaborators present possible explanation in a recent paper "The Formation of a 70 Msun Black Hole at High Metallicity".
There are though works questioning the interpretation of a companion in LB-1 system as a 70 solar masses black hole:
"Weighing in on black hole binaries with BPASS: LB-1 does not contain a 70M⊙ black hole"
"No signature of the orbital motion of a putative 70 solar mass black hole in LB-1"
"Not so fast: LB-1 is unlikely to contain a 70 M⊙ black hole".
Pictures show artistic vision of the system, it's size and location in the Galaxy.
