ASCA observation of Cygnus X-1 in the soft state: Mass of the compact object

We describe the ASCA observation of the black hole candidate Cyg X-l in the soft state of 1996 May and report the mass estimation of the central object from X-ray spectral analysis. The energy spectrum in 0.7-10 keV can be described by the sum of a power law (Gamma = 2.3 +/- 0.1) and an ultrasoft component; the latter is most probably the emission from an optically thick accretion disk. Through the spectral analysis of the ultrasoft component, we estimated the size of the accretion disk, which is closely related to the Schwarzschild radius and, hence, to the mass of the central object. Taking the general relativistic effects into account, the most probable mass of the compact object is estimated as 12(-1)(+3) M.. This value is independent of, yet consistent with, the mass estimate based on the orbital kinematics information.