Observational Study of the Cosmic X-ray Background with the Scientifica Satellite ASTRO-D

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The objective of this scientific project is to investigate the cosmic X-ray background (CXB), using the scientific satellite ASTRO-D (ASCA) launched on 1993 February 20, and to carry out in-orbit calibration of the GIS (Gas Imaging Spectrometer) instrument onboard ASCA.The CXB is anenigmatic phenomenon known science the birth of the X-ray astronomy, but its exact origin is still unknown. Through three years of investigation, the following achievements have been obtained.1. The in-orbit performance verification and detailed calibration of the GIS have been accomplished. In particular, the GIS particle background has been precisely quantified, which is important for the investigation of diffuse emissions including the CXB.2. By taking into account the point spread function and stray light of the X-ray telescope, a new scheme of data analysis has been developed for application to widely extended X-ray sources.3. New insight has been obtained for various extra-Galactic objects which potentially contribute to the CXB.In particular ; (1) importance of the large-scale magnetic fields has been revealed ; (2) the dark matter distribution has been found to exhibit galaxy/cluster tierarchy ; and (3) a number of low-luminosity active nuclei have been discovered in nearby normal galaxies.4. The CXB spectrum has been show to exhibit a single power-law shape with a photon index 1.4 over the 1-10 keV energy range. This contradicts the previous results obtained with the ROSAT satellite. In addition, after removing point sources brighter than 3*10^<-13> erg/s/cm^2 (in 2-10 keV), the field-to field fluctuation of te CXB surface brightness is confirmed to be <plus-minus>3% at most. These results give strong constraints to the models that attempt to synthesize the CXB as a superposition of distant objects.