海老沢 研

We present the results of 4 simultaneous observations of Cygnus X-1 by Ginga<br /> and OSSE. The X-ray/gamma-ray spectra can be described by an intrinsic<br /> continuum and a component due to Compton reflection including an iron K-alpha<br /> line. The intrinsic spectrum at X-ray energies is a power-law with a photon<br /> spectral index of Gamma=1.6. The intrinsic gamma-ray spectrum can be<br /> phenomenologically described by either a power-law without cutoff up to 150<br /> keV, and an exponential cutoff above that energy, or by an exponentially cutoff<br /> power law and a second hard component.

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 ∼ 198 (or possibly ∼ 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 ∼ 9keV. 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.