2007PASJ...59S.245K

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 <FONT SIZE="-2">I</FONT> Kα at 6.4keV and K-shell absorption edge at 7.1keV, which are from neutral and/or low ionization states of iron, and the K-shell lines at 6.7keV and 6.9keV from He-like (Fe <FONT SIZE="-2">XXV</FONT> Kα) and hydrogenic (Fe <FONT SIZE="-2">XXVI</FONT> Lyα) ions of iron. In addition, Kα lines from neutral or low ionization nickel (Ni <FONT SIZE="-2">I</FONT> Kα) and He-like nickel (Ni <FONT SIZE="-2">XXVII</FONT> Kα), Fe <FONT SIZE="-2">I</FONT> Kb, Fe <FONT SIZE="-2">XXV</FONT> Kb, Fe <FONT SIZE="-2">XXVI</FONT> Lyb, Fe <FONT SIZE="-2">XXV</FONT> Kg, and Fe <FONT SIZE="-2">XXVI</FONT> Lyg were detected for the first time. The line center energies and widths of Fe <FONT SIZE="-2">XXV</FONT> Kα and Fe <FONT SIZE="-2">XXVI</FONT> Lyα favor a collisional excitation plasma for the origin of the GCDX. The electron temperature determined from the line flux ratio of Fe <FONT SIZE="-2">XXV</FONT> Kα/ Fe <FONT SIZE="-2">XXV</FONT> Kb is similar to the ionization temperature determined from that of Fe <FONT SIZE="-2">XXV</FONT> Kα/ Fe <FONT SIZE="-2">XXVI</FONT> Lyα. Thus it would appear that the GCDX plasma is close to ionization equilibrium. The 6.7keV 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.