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 $|b| \gt 10^\circ$ 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 $\sim 2\times 10^{-13} \,\mathrm{erg} \,\mathrm{cm}^{-2} \,\mathrm{s}^{-1}$ (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 $\sim 2 \,\mathrm{keV}$, which could be expressed by an additional thermal model with $kT\simeq 0.4 \,\mathrm{keV}$ or a steep power-law model with a photon index of $\Gamma^\mathrm{soft}\simeq 6$. The 0.5–2 keV intensities of the soft thermal component varied significantly from field to field 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 $\Gamma^\mathrm{hard} = 1.412\pm 0.007\pm 0.025$ and the average 2–10 keV intensity was calculated as $F_\mathrm{X}^\mathrm{hard} = (6.38\pm 0.04\pm 0.64)\times 10^{-8} \,\mathrm{erg} \,\mathrm{cm}^{-2} \,\mathrm{s}^{-1} \,\mathrm{sr}^{-1}$ ($1\,\sigma$ 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 $6.49^{+0.56}_{-0.61}\%$, 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. ($\sim 0.5 \,\mathrm{deg}^2$), even assuming a single $\log{N} \hbox{--} \log{S}$ relation on the whole sky. Based on the observed fluctuation and the absolute intensity, an acceptable region of the $\log{N} \hbox{--} \log{S}$ 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 ($\Gamma_\mathrm{S}\simeq 1.1\pm 1.0$).