Faint X‐Ray Sources Resolved in the <i>ASCA</i> Galactic Plane Survey and Their Contribution to the Galactic Ridge X‐Ray Emission

Abstract

The X-ray emission from the central region of the Galactic plane, |l|<45 deg and |b|<0.4 deg, was studied in the 0.7-10 keV energy band with a spatial resolution of ~3' with the 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 resolved 163 discrete sources with a 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 ~1 deg. The longitudinal variation of the GRXE in the energy band above 4 keV shows a large enhancement within |l|<30 deg. This suggests a strong enhancement of X-ray emissivity inside the 4-kpc arms. Searches for identifications of the resolved 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. We classified the sources into several groups according to the spectra and analyzed the spectra summed within each group. Possible X-ray origins of these sources are discussed based on the grouping analysis. Also, we derived the LogN-LogS relations of the resolved sources in the energy bands below and above 2 keV. The obtained LogN-LogS relation of the Galactic X-ray sources above 2 keV is represented by a power-law with an index of -0.79+/-0.07. This flat LogN-LogS relation suggests that the spatial distribution of the sources should have an arm-like 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.