The Galactic ridge X-ray emission (GRXE) is an enhanced X-ray emission along the Galactic plain, whose origin still remains unknown. The GRXE was studied for the first time in the 0.5-10 keV energy band with a spatial resolution of 3' by the ASCA Galactic plane survey. We analyzed the latest survey data which covers the spatial area of -60° < l < 50° and |b| < 0.4° almost uniformly.
We determined the large scale distribution of the GRXE after subtracting discrete X-ray sources with a flux above 10-12 ergs cm-2 s-1 and revealed that the volume emissivity of the GRXE is highly concentrated within the 4 kpc arm.
We performed a discrete source survey to resolve 143 sources in total, and obtained LogN-LogS relations for these sources. In the soft band of 0.7-2 keV, the slope of the LogN-LogS relation is -0.90±0.15, which is consistent with those of the previous Einstein and ROSAT surveys. On the other hand, in the hard band of 2-10 keV, the slope of the LogN-LogS relation is -0.82±0.09, significantly flatter than -1.0. Considering a scale height of the Galactic plane covered by the ASCA Galactic plane surveys, we conclude that the slope of the LogN-LogS relation suggests that the relation represents the luminosity function of the sources rather than the spatial distribution, that the spatial distribution of the sources has a scale height much smaller than 100 pc, and/or that they are distributed in arm-like structures.
We performed temporal and spectral analyses of the resolved X-ray sources which were detected with a significance level more than 5σ. We classified the sources into six subgroups according to the power-law photon index and the absorption column density. From the average spectra of the groups, we found that two groups of sources show average spectra similar to a spectrum of the GRXE. They show a strong Fe K-shell emission line with an equivalent width of about 1 keV. We consider magnetic cataclysmic variables (CVs) to be likely candidates of these sources, from the spectra, the luminosity, and the spatial number density.
We analyzed the small-scale spatial intensity fluctuation of the GRXE after subtracting the large scale variations and the contributions of resolved discrete sources. The small-scale fluctuation is significant compared to the photon-counting Poisson fluctuations. However the amplitude can be explained by the fluctuation of the cosmic X-ray background coming through the Galactic absorption in the 2-10 keV band. From this we obtained an upper limit on the small-scale spatial fluctuation of the GRXE, which strongly constrains the flux and the number density of discrete sources if we are to explain the GRXE with a sum of discrete sources; more than 107 sources with luminosities smaller than 1030 ergs s-1 must exist in the Galaxy. This source number density is larger by three orders of magnitude than that of CVs in the solar neighborhood. Such a large number for discrete sources is difficult to explain.