Ken Ebisawa

The in-orbit performance and calibration of the Gas Imaging Spectrometer (GIS), located on the focal plane of the X-ray astronomy satellite ASCA, are described. An extensive in-orbit calibration has confirmed its basic performance, including a position resolution of 0.6 mm (FWHM) and an energy resolution of 7.8% (FWHM), both at 6 keV. When combined with the X-ray telescope, the GIS sensitivity range becomes 0.7-10 keV. The in-orbit non X-ray background of the GIS has been confirmed to be as low as (5-9) × 10-4 c s-1 cm-2 keV-1 over the 1-10 keV range. The long-term detector gain has been stable within a few % for nearly 3 years. Extensive observations of the Crab Nebula and other sources have provided accurate calibrations of the position response, photometric capability, dead time, and timing accuracy of the GIS. Furthermore, the overall energy response, including the temporal and positional gain variations and the absolute gain scale, has been calibrated to ∼ 1%. Thus, the GIS is working as an all-round cosmic X-ray detector, capable of X-ray imagery, fine X-ray spectroscopy, X-ray photometry with a flux dynamic range covering more than 5 orders of magnitude, and fast X-ray photometry with a time resolution up to 60 μs.

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.