Weighing stellar - mass black holes with Gaia
  Stellar-mass black holes have been detected by radial-velocity observations in star/black hole binaries. These allow only the determination of the mass function. Tracking the astrometric orbits of the visible components of the star/black hole binaries would allow the full determination of the black hole masses, which would be of great astrophysical interest. We investigate the possibilities to do this with Gaia. A very promising object seems to be Cyg X-1. The donor star is an O9.7Iab supergiant having a mass of 25 M_Sun, and the mass of the black hole is estimated to be at least 13 M_Sun masses. The period of 5.6 days implies a semi-major axis of 1.01 micropc of the binary orbit. At the distance of Cyg X-1 (2.5kpc) this translates into a semi-major axis of about 28 microarcsec for the visible component, which can be detected by Gaia. We discuss further candidates. 
B. Fuchs, U. Bastian