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-raybackground with the ASCA GIS based on the ASCA MSS and LSS observations. Atotal of 91 fields with Galactic latitude |b|>10 deg were selected with a skycoverage of 50 deg^2 and 4.2 Ms of exposure. For each field, sources brighterthan \sim 2\times 10^{-13}\ergs (2-10 keV) were eliminated. Spectral fits witha single power-law model for the individual 0.7-10 keV spectra showed asignificant excess below \sim 2 keV, which could be expressed by an additionalthermal model with kT\simeq 0.4 keV or a steep power-law model with photonindex GS\simeq 6. The 0.5-2 keV intensities of the soft thermal componentvaried significantly by 1\sigma=52^{+4}_{-5}%, and showed a maximum toward theGalactic Center. This component is considered to be entirely Galactic. As forthe hard power-law component, an average photon index of 91 fields was obtainedto be GH = 1.412\pm 0.007\pm 0.025 and the average 2-10 keV intensity wascalculated as FH = (6.38\pm 0.04\pm 0.64)\times 10^{-8} \ergss. The Galacticcomponent is marginally detected in the hard band. The 2-10 keV intensitiesshows a 1\sigma deviation of 6.49^{+0.56}_{-0.61}%, while deviation due to theNXB is 3.2%. The observed deviation can be explained by the Poisson noise ofsource count in the f.o.v. (\sim 0.5 deg^2), even assuming a single \lognrelation on the whole sky. Based on the observed fluctuation and the absoluteintensity, an acceptable region of the \logn relation was derived, showing aconsistent feature with the recent Chandra and XMM-Newton results. Fluctuationof the spectral index was also examined, and it implied a large amount of hardsources and a substantial variation in the intrinsic source spectra (GS\simeq1.1\pm 1.0).Comment: 27 pages, 20 figures, accepted for publication in PASJ, amended figure8 and figure1