Spectral Study of the Galactic Ridge X-Ray Emission with Suzaku

Abstract

Abstract In order to carry out a precise spectral study of the Galactic Ridge X-ray Emission using Suzaku, we have observed a typical Galactic plane field at $(l,b) =$ (28D46, $-$0D20), which is already deeply observed with Chandra, and known to be devoid of bright X-ray point sources. Thanks to the low background and high spectral resolution of Suzaku, we were able to resolve three narrow iron K-emission lines from low-ionized (6.41keV), helium-like (6.67keV), and hydrogenic ions (7.00keV). The cosmic-ray ion charge-exchange model or the non-equilibrium ionization plasma model are unlikely to explain these line features, since they require either broad emission lines or lines at intermediate ionization states. Collisional ionization equilibrium plasma is the likely origin for the 6.67keV and 7.00keV lines; however, the origin of the 6.41keV line, which is due to fluorescence from cold material, has not been elucidated. We could also precisely measure the absolute X-ray surface brightness in the direction of the Galactic plane. Excluding point sources brighter than $\sim$2 $\times 10^{-13}$ergs$^{-1}$cm$^{-2}$ (2-10keV), the total surface brightness on the Galactic plane is $\sim$6.1 $\times 10^{-11}$ergs$^{-1}$cm$^{-2}$deg$^{-2}$ (2-10keV), including the contribution of the cosmic X-ray background, which is estimated to be $\sim$1.3$\times 10^{-11}$ergs$^{-1}$cm$^{-2}$deg$^{-2}$.