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Date and Time of the Query: 2019-06-15 T21:02:01 PDT
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For refcode 2002PASJ...54..327K:
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Copyright by Astronomical Society of Japan. 2002PASJ...54..327K Study of the X-Ray Background Spectrum and Its Large-Scale Fluctuation with ASCA Akihiro KUSHINO, Yoshitaka ISHISAKI, Umeyo MORITA, Noriko Y. YAMASAKI, Manabu ISHIDA, Takaya OHASHI, and Yoshihiro UEDA National Space Development Agency of Japan (NASDA), 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505 kushino.akihiro@nasda.go.jp Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397 The Institute of Space and Astronautical Science (ISAS), 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510 (Received 2002 January 7; accepted 2002 April 9) 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^deg^ 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 ~ 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 ~ 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 {GAMMA}^soft^ ~ 6. The 0.5-2 keV intensities of the soft thermal component varied significantly from field to field by 1 {sigma} = 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 F_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. (~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-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}_S_ ~ 1.1 +/- 1.0). Key Words: cosmology: diffuse radiation - galaxies: active - surveys - X-rays: diffuse background - X-rays: general
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