Iron and Nickel Line Diagnostics for the Galactic Center Diffuse Emission

Abstract

We have observed the diffuse X-ray emission from the Galactic Center (GC) using the X-ray Imaging Spectrometer (XIS) on Suzaku. The high-energy resolution and the low-background orbit provided excellent spectra of the GC diffuse X-rays (GCDX). The XIS found many emission lines in the GCDX near the energy of K-shell transitions of iron and nickel. The most pronounced features are Fe I $\mathrm{K}\alpha$ at 6.4 keV and K-shell absorption edge at 7.1 keV, which are from neutral and/or low ionization states of iron, and the K-shell lines at 6.7 keV and 6.9 keV from He-like (Fe XXV $\mathrm{K}\alpha$) and hydrogenic (Fe XXVI Ly$\alpha$) ions of iron. In addition, $\mathrm{K}\alpha$ lines from neutral or low ionization nickel (Ni I $\mathrm{K}\alpha$) and He-like nickel (Ni XXVII $\mathrm{K}\alpha$), Fe I $\mathrm{K}\beta$, Fe XXV $\mathrm{K}\beta$, Fe XXVI $\mathrm{Ly}\beta$, Fe XXV $\mathrm{K}\gamma$, and Fe XXVI $\mathrm{Ly}\gamma$ were detected for the first time. The line center energies and widths of Fe XXV $\mathrm{K}\alpha$ and Fe XXVI $\mathrm{Ly}\alpha$ favor a collisional excitation plasma for the origin of the GCDX. The electron temperature determined from the line flux ratio of Fe XXV $\mathrm{K}\alpha$$/$ Fe XXV $\mathrm{K}\beta$ is similar to the ionization temperature determined from that of Fe XXV $\mathrm{K}\alpha$$/$ Fe XXVI $\mathrm{Ly}\alpha$. Thus it would appear that the GCDX plasma is close to ionization equilibrium. The 6.7 keV flux and temperature distribution to the galactic longitude is smooth and monotonic, in contrast to the integrated point source flux distribution. These facts support the hypothesis that the GCDX is truly diffuse emission rather than the integration of the outputs of a large number of unresolved point sources. In addition, our results demonstrate that the chemical composition of Fe in the interstellar gas near the GC is constrained to be about 3.5 times solar abundance.