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 instrument based on the ASCA Medium Sensitivity Survey and Large Sky Survey observations. A total of 91 fields with Galactic latitude |b| 10^ were selected with a sky coverage of 50 deg² and 4. 2 Ms of exposure. For each field, non-X-ray events were carefully subtracted and sources brighter than 2 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 2 \, keV, which could be expressed by an additional thermal model with kT 0. 4 \, keV or a steep power-law model with a photon index of ˢoft 6. The 0. 5–2 keV intensities of the soft thermal component varied significantly from field to field by 1\, = 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 ʰard = 1. 412 0. 007 0. 025 and the average 2–10 keV intensity was calculated as FXʰard = (6. 38 0. 04 0. 64) 10^-8 \, erg \, cm^-2 \, s^-1 \, sr^-1 (1\, statistical and systematic errors). The Galactic component is marginally detected in the hard band. The 2–10 keV intensities show a 1\, deviation of 6. 49^+0. 56-₀. ₆₁\%, 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. (0. 5 \, deg²), even assuming a single N -- S relation on the whole sky. Based on the observed fluctuation and the absolute intensity, an acceptable region of the N -- S relation was derived, showing a consistent feature with the recent Chandra and XMM-Newton 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 (S 1. 1 1. 0).