2002/04/30 by Akihiro Kushino, Yoshitaka Ishisaki, Umeyo Morita +4 · 1 citation
Physics and Astronomy · #astro-ph
paper · pdf · doi:10.1093/pasj/54.3.327
27 pages, 20 figures, accepted for publication in PASJ, amended figure8 and figure17
arxiv created 2002/05/12 · arxiv updated 2015/06/24
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 deg2 and 4.2 Ms of exposure. For each field, sources brighter than ∼ 2× 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 ∼ 2 keV, which could be expressed by an additional thermal model with kT≃ 0.4 keV or a steep power-law model with photon index GS≃ 6. The 0.5-2 keV intensities of the soft thermal component varied significantly by 1σ=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± 0.007± 0.025 and the average 2-10 keV intensity was calculated as FH = (6.38± 0.04± 0.64)× 10-8 \ergss. The Galactic component is marginally detected in the hard band. The 2-10 keV intensities shows a 1σdeviation 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. (∼ 0.5 deg2), 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≃ 1.1± 1.0).