Study of the X-Ray Background Spectrum and its Large-Scale Fluctuation with ASCA

Abstract

We studied the energy spectrum and the large-scale fluctuation of the X-ray background with the ASCA GIS based on the ASCA MSS and LSS observations. A total of 91 fields with Galactic latitude |b|>10 deg were selected with a sky coverage of 50 deg^2 and 4.2 Ms of exposure. For each field, sources brighter than \sim 2\times 10^{-13}\ergs (2-10 keV) were eliminated. Spectral fits with a single power-law model for the individual 0.7-10 keV spectra showed a significant excess below \sim 2 keV, which could be expressed by an additional thermal model with kT\simeq 0.4 keV or a steep power-law model with photon index GS\simeq 6. The 0.5-2 keV intensities of the soft thermal component varied significantly by 1\sigma=52^{+4}_{-5}%, and showed a maximum toward the Galactic Center. This component is considered to be entirely Galactic. As for the hard power-law component, an average photon index of 91 fields was obtained to be GH = 1.412\pm 0.007\pm 0.025 and the average 2-10 keV intensity was calculated as FH = (6.38\pm 0.04\pm 0.64)\times 10^{-8} \ergss. The Galactic component is marginally detected in the hard band. The 2-10 keV intensities shows a 1\sigma deviation of 6.49^{+0.56}_{-0.61}%, while deviation due to the NXB is 3.2%. The observed deviation can be explained by the Poisson noise of source count in the f.o.v. (\sim 0.5 deg^2), even assuming a single \logn relation on the whole sky. Based on the observed fluctuation and the absolute intensity, an acceptable region of the \logn relation was derived, showing a consistent feature with the recent Chandra and XMM-Newton results. Fluctuation of the spectral index was also examined, and it implied a large amount of hard sources and a substantial variation in the intrinsic source spectra (GS\simeq 1.1\pm 1.0).