Iron and Nickel Line Diagnostics for the Galactic Center Diffuse Emission

We have observed the diffuse X-ray emission from the Galactic center (GC)using the X-ray Imaging Spectrometer (XIS) on Suzaku. The high-energyresolution and the low-background orbit provide excellent spectra of the GCdiffuse X-rays (GCDX). The XIS found many emission lines in the GCDX near theenergy of K-shell transitions of iron and nickel. The most pronounced featuresare FeI K alpha at 6.4 keV and K-shell absorption edge at 7.1 keV, which arefrom neutral and/or low ionization states of iron, and the K-shell lines at 6.7keV and 6.9 keV from He-like (FeXXV K alpha) and hydrogenic (FeXXVI Ly alpha)ions of iron. In addition, K alpha lines from neutral or low ionization nickel(NiI K alpha) and He-like nickel (NiXXVII K alpha), and FeI K beta, FeXXV Kbeta, FeXXVI Ly beta, FeXXV K gamma and FeXXVI Ly gamma are detected for thefirst time. The line center energies and widths of FeXXV K alpha and FeXXVI Lyalpha favor a collisional excitation (CE) plasma for the origin of the GCDX.The electron temperature determined from the line flux ratio of FeXXV K alpha /FeXXV K beta is similar to the ionization temperature determined from that ofFeXXV K alpha /FeXXVI Ly alpha. Thus it would appear that the GCDX plasma isclose to ionization equilibrium. The 6.7 keV flux and temperature distributionto the galactic longitude is smooth and monotonic,in contrast to the integratedpoint source flux distribution. These facts support the hypothesis that theGCDX is truly diffuse emission rather than the integration of the outputs of alarge number of unresolved point sources. In addition, our results demonstratethat the chemical composition of Fe in the interstellar gas near the GC isconstrained to be about 3.5 times solar.Comment: 11 pages, 19 figures. Accepted for publication in PASJ Suzaku Special Issue (vol. 59 sp. 1