James Miller-Jones

The high-mass X-ray binary system Cygnus X-1 contains one of the first known and best-studied black holes. We recently refined the distance to this important source using astrometric very long baseline interferometry observations. The new distance implied that the masses of the two binary components both needed to be revised upwards, with our modelling showing that the system hosts the most massive known stellar-mass black hole in our Galaxy. The formation of a 21 solar-mass black hole in a Galactic environment with roughly solar metallicity challenges existing model prescriptions for the wind mass loss rates from massive stars. I will discuss our new measurements and their consequences for the likely formation and subsequent evolutionary pathway of the black hole, and briefly touch on the broader implications of this measurement.