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A common idea for the origin of the Galactic diffuse X-ray emission,particularly that of the iron lines from neutral and highly ionized atoms, is asuperposition of many cataclysmic variables and coronally active binaries. Inthis scenario, the flux should symmetrically distribute between the east andwest on the plane with respect to Sagittarius A$^*$ because the stellar massdistribution determined by infrared observations is nearly symmetric. Thissymmetry is confirmed for the highly ionized iron line as well as the continuumemission. However, a clear excess of the neutral iron line in the near east ofthe Galactic center compared to the near-west side is found. The fluxdistribution of the excess emission well correlates with molecular columndensity. The X-ray spectrum of the excess emission is described by a power-lawcontinuum plus a 6.4 keV line with the large equivalent width of $\sim1.3$ keV,which is hardly explained by the low-energy electron bombardment scenario. Thelongitudinal and latitudinal distribution of the excess emission disfavors theX-ray irradiation, neither by Sagittarius A$^*$ nor by nearby X-ray binaries.Then the low-energy proton bombardment is the most probable origin although thehigh energy density $\sim 80$ eV cm$^{-3}$ in 0.1-1000 MeV is required andthere is no conventional proton source in the vicinity.

Enhancement of the 6.4 keV line in the inner Galactic ridge: Proton-induced fluorescence?