SPECTRAL VARIATIONS OF LMC X-3 OBSERVED WITH GINGA

The prime black hole candidate LMC X-3 was observed over 3 yr with the Ginga satellite, and a characteristic spectral variation was found accompanying the periodic intensity variation of approximately 198 (or possibly approximately 99) days (Cowley et al. 1991). The energy spectrum of LMC X-3 consists of the soft, thermal component and the hard, power-law component, which are respectively dominant below and above approximately 9 keV. The soft component, which carries most of the X-ray intensity, shows a clear correlation between the intensity and the hardness, while the hard component varies independently of the soft component. It was found that the spectral variation of the soft component is well described by an optically thick accretion disk model with a remarkably constant innermost radius and variable mass accretion rate. The constancy of the innermost radius suggests it is related to the mass of the central object. Supposing that the innermost radius of the accretion disk is the last stable orbit around a black hole, i.e., 3 times the Schwarzschild radius, we estimate the mass of the central object in LMC X-3. Satisfactory agreement has been found between the mass thus derived and the fiducial mass lower limit determined from observations of the binary motion.