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Faint X-Ray Sources Resolved in the ASCA Galactic Plane Survey and Their Contribution to the Galactic Ridge X-Ray Emission
The X-ray emission from the central region of the Galactic plane,|l|<~45° and |b|<~0.4d, was studied in the 0.7-10 keV energyband with a spatial resolution of ~3' with the Advanced Satellite forCosmology and Astrophysics (ASCA) observatory. We developed a newanalysis method for the ASCA data to resolve discrete sources from theextended Galactic ridge X-ray emission (GRXE). We successfully resolved163 discrete sources with an X-ray flux down to 10-12.5 ergscm-2 s-1 and determined the intensity variationsof the GRXE as a function of the Galactic longitude with a spatialresolution of about 1°. The longitudinal intensity variation in theenergy band above 4 keV, for which there is little absorption in theGalactic plane, shows a large enhanced feature within |l|<~30°.This suggests a strong enhancement of X-ray emissivity of the GRXEinside the 4 kpc arm of the Galaxy. Searches for identifications of theresolved X-ray sources with cataloged X-ray sources and optical starsshow that the 66% are unidentified. Spectral analysis of each sourceshows that a large number of the unidentified sources have hard X-rayspectra obscured by the Galactic interstellar medium. We classified thesources into several groups by the flux, the hardness and the softnessof the spectra, and performed further detailed analysis for the spectrasummed within each group. Possible candidates of X-ray origins of theseunidentified sources are discussed based on the grouping spectralanalysis. Also, we derived the logN-logS relations of the resolvedsources in the energy bands below and above 2 keV separately. ThelogN-logS relation of the Galactic X-ray sources above 2 keV wasobtained for the first time with this study. It is represented by apower-law with an index of -0.79+/-0.07 after correction for thecontribution of extragalactic X-ray sources. This flat power-lawrelation suggests that the spatial distribution of the X-ray sourcesshould have an armlike structure in which the solar system is included.The integrated surface brightness of the resolved sources is about 10%of the total GRXE in both energy bands. The approximately 90% of theemission remaining is still unresolved.
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