Origin of the Galactic ridge X-ray emission

Origin of the X-ray emission along the Galactic ridge is still an unresolve d issue. The ASCA spectra show emission lines of various elements, suggesti ng thermal emission of thin hot plasmas in non-equilibrium ionization. If t hey are Galactic-scale diffuse plasmas, their energy density is too high to be contained in the Galactic plane. The energy loss rate due to a conseque nt plasma outflow is similar to 10(43) erg s(-1) for which no energy source is readily found. Despite a large difference in brightness, the spectrum of the emission from the Galactic center region is found to be strikingly similar, essentially identical, to that of the Galactic ridge emission. Because of the same phys ical properties, the emissions from both regions are most probably of a sin gle origin. Then, the interpretation in terms of multiple diffuse plasma co mponents of non-equilibrium ionization seems implausible, since there is no reason that the degree of ionization is the same for these two regions of orders of magnitude different electron densities. In view of these problems in the diffuse plasma origin, we revisit the poss ibility of discrete source origin, with use of the ROSAT archival PSPC data for the Galactic ridge. In total 19 sources are detected in the 0.7-2 keV range above a threshold of similar to 10(-14) erg cm(-2) s(-1) in 0.5-2 keV , corresponding to a surface density of similar to 10(5) sources sr(-1). Ba sed on a model calculation, the observed result requires a class of hard so urce with L-X similar to 10(29-30) erg s(-1) and as abundant as the coronal sources to account for the ridge emission. Such a class of source is not k nown. We also consider a possibility of non-thermal origin of the Galactic ridge emission, in which the X-ray emission lines are produced as a result of cha rge exchange interactions of cosmic-ray heavy ions and the continuum is an extension of the soft gamma-ray component to the X-ray range.