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.°4, was studied in the 0.7-10 keV energy band with a spatial resolution of ∼3′ with the Advanced Satellite for Cosmology and Astrophysics (ASCA) observatory. We developed a new analysis method for the ASCA data to resolve discrete sources from the extended Galactic ridge X-ray emission (GRXE). We successfully resolved 163 discrete sources with an X-ray flux down to 10-12.5 ergs cm-2 s-1 and determined the intensity variations of the GRXE as a function of the Galactic longitude with a spatial resolution of about 1°. The longitudinal intensity variation in the energy band above 4 keV, for which there is little absorption in the Galactic plane, shows a large enhanced feature within |l| ≲ 30°. This suggests a strong enhancement of X-ray emissivity of the GRXE inside the 4 kpc arm of the Galaxy. Searches for identifications of the resolved X-ray sources with cataloged X-ray sources and optical stars show that the 66% are unidentified. Spectral analysis of each source shows that a large number of the unidentified sources have hard X-ray spectra obscured by the Galactic interstellar medium. We classified the sources into several groups by the flux, the hardness and the softness of the spectra, and performed further detailed analysis for the spectra summed within each group. Possible candidates of X-ray origins of these unidentified sources are discussed based on the grouping spectral analysis. Also, we derived the log N-log S relations of the resolved sources in the energy bands below and above 2 keV separately. The log N-log S relation of the Galactic X-ray sources above 2 keV was obtained for the first time with this study. It is represented by a power-law with an index of -0.79 ± 0.07 after correction for the contribution of extragalactic X-ray sources. This flat power-law relation suggests that the spatial distribution of the X-ray sources should 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 the emission remaining is still unresolved.