* Astronomy

Title: The Mass of the Black Hole in Cygnus X-1
Authors: Jerome A. Orosz, Jeffrey E. McClintock, Jason P. Aufdenberg, Ronald A. Remillard, Mark J. Reid, Ramesh Narayan, Lijun Gou

Cygnus X-1 is a binary star system that is comprised of a black hole and a massive giant companion star in a tight orbit. Building on our accurate distance measurement reported in the preceding paper, we first determine the radius of the companion star, thereby constraining the scale of the binary system. Then, by modelling an extensive collection of optical data, we find a mass for the companion of M_{opt}=19.2±1.9 solar masses. Of central importance, we further determine the mass of the black hole, M =14.8±1.0 solar masses, and the angle of inclination of the orbital plane to our line of sight, i=27.1±0.8 deg. In the following paper, we use these precise values of M and i, and our secure value of the distance D, to determine the spin of the black hole.

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