Suzaku spectral study of the Galactic Ridge X-ray Emission

We have observed a typical Galactic plane blank field with Suzaku for 100 ksec to carry out spectral study of the Galactic Ridge X-ray Emission (GRXE). The field had been previously studied with Chandra, and is known to be devoid of bright X-ray point sources. We were able to resolve, for the first time, three narrow iron K-emission lines from low-ionized (6.4 keV), helium-like (6.67 keV), and hydrogenic ions (7.0 keV). These line features constrain the GRXE emission mechanisms. The cosmic-ray ion charge exchange model or the non-ionization equilibrium (NIE) plasma model are very unlikely, since they require either broad emission lines or lines at intermediate ionization states. We were able to precisely measure the absolute GRXE flux thanks to the low background and small stray-light contamination of Suzaku. Excluding point sources brighter than 2x10-13 ergs s-1 cm-2 (2-10 keV), only ∼10% of the 2-10 keV GRXE flux is explained by the Chandra point sources brighter than 3x10-15 ergs s-1 cm-2. (author)