Origin of the Galactic ridge X‐ray emission

Origin of the X‐ray emission along the Galactic ridge is still an unresolved issue. The ASCA spectra show emission lines of various elements,suggesting thermal emission of thin hot plasmas in non‐equilibrium ionization. If they 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 consequent plasma outflow is ∼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 physical properties, the emissions from both regions are most probably of a single origin. Then, the interpretation in terms of multiple diffuse plasma components 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 possibility 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 ∼10 —14 erg cm —2 s —1 in 0.5—2 keV, corresponding to a surface density of ∼10 5 sources sr —1 . Basedon a model calculation, the observed result requires a class of hardsource with L X ∼ 10 29—30 erg s—1 andas abundant as the coronal sources to account for the ridge emission. Such a class of source is not known.