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\ less than or similar to 45 degrees and \b\ less than or similar to 0.degrees4, w as studied in the 0.7-10 keV energy band with a spatial resolution of simil ar to3' with the Advanced Satellite for Cosmology and Astrophysics (ASCA) o bservatory. We developed a new analysis method for the ASCA data to resolve discrete sources from the extended Galactic ridge X-ray emission (GRXE). W e 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 GRX E as a function of the Galactic longitude with a spatial resolution of abou t 1 degrees. 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\ less than or similar to 30 degrees. This suggests a strong enhancement ment of X-ray emissivity of the GRXE inside t he 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 num ber of the unidentified sources have hard X-ray spectra obscured by the Gal actic interstellar medium. We classified the sources into several groups by the flux, the hardness and the softness of the spectra, and performed furt her detailed analysis for the spectra summed within each group. Possible ca ndidates of X-ray origins of these unidentified sources are discussed based on the grouping spectral analysis. Also, we derived the log N-log S relati ons of the resolved sources in the energy bands below and above 2 keV separ ately. The log N-log S relation of the Galactic X-ray sources above 2 keV w as obtained for the first time with this study. It is represented by a powe r-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 struc ture in which the solar system is included. The integrated surface brightne ss of the resolved sources is about 10% of the total GRXE in both energy ba nds. The approximately 90% of the emission remaining is still unresolved.