Study of the X-ray background spectrum and its large-scale fluctuation with ASCA

We studied the energy spectrum and the large-scale fluctuation of the X-ray background with the ASCA GIS instrument based on the ASCA Medium Sensitivity Survey and Large Sky Survey observations. A total of 91 fields with Galactic latitude \b\ > 10degrees were selected with a sky coverage of 50 deg(2) and 4.2 Ms of exposure. For each field, non-X-ray events were carefully subtracted and sources brighter than similar to 2 x 10-(13) erg cm(-2) s(-1) (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 similar to 2 keV, which could be expressed by an additional thermal model with kT similar or equal to 0.4 keV or a steep power-law model with a photon index of Gamma(soft) similar or equal to 6. The 0.5-2 keV intensities of the soft thermal component varied significantly from field to field by 1sigma = 52(-5)(+4%), 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 Gamma(hard) = 1.412 +/- 0.007 +/- 0.025 and the average 2-10 keV intensity was calculated as Gamma(X)(hard) = (6.38 +/- 0.04 +/- 0.64) x 10(-8) erg cm(-2) s(-1) sr(-1) (1 sigma statistical and systematic errors). The Galactic component is marginally detected in the hard band. The 2-10 keV intensities show a 1 sigma deviation of 6.49(-0.61)(+0.56%), while deviation due to the reproducibility of the particle background is 3.2%. The observed deviation can be explained by the Poisson noise of the source count in the f.o.v. (similar to 0.5 deg(2)), even assuming a single log N-log S relation on the whole sky. Based on the observed fluctuation and the absolute intensity, an acceptable region of the log N-log S relation was derived, showing a consistent feature with the recent Chandra and XMM-Nelvton results. The fluctuation of the spectral index was also examined; it implied a large amount of hard sources and a substantial variation in the intrinsic source spectra (Gamma(S) similar or equal to 1.1 +/- 1.0).