Suzaku Observations of the Galactic Center: The Origin of Highly Ionized Iron Lines

Abstract

We have made long exposure observations ( 134 hours in total ) of the Galactic Center (GC) region with Suzaku, the 5th Japanese X-ray satellite. With the large effective area, high energy resolution, and low background of X-ray Imaging Spectrometers (XISs; CCD cameras on board Suzaku), we resolved the line complex near the energy of K-shell transitions of iron into 3 Kα lines, 6.4 keV (neutral), 6.7 keV (He-like) and 6.9 keV (H-like), and discovered 6 iron and nickel lines above 7 keV. The center energy of the He-like iron is determined very accurately to be 6680 ± 1 ± 5 eV, which favors the hot plasma origin with the predicted center energy of 6685 eV, rather than the charge exchange origin of 6666 eV. The line fluxes are uniformly distributed, in contrast to the clumpy structure of the molecular clouds. This fact is also against the charge exchange origin, because the flux of this process should trace the molecular clouds where donor electrons (neutral hydrogen) are rich. The electron temperature and ionization temperature are determined with the intensity ratio of the 7.9/6.7 keV and 6.9/6.7 keV lines, and are consistent with 6.5 keV, indicating a collisional ionization equilibrium plasma. We hence conclude that the origin of highly ionized iron lines is a hot thin plasma. We discovered five new extended objects from the GC region. They are probably supernova remnants or X-ray reflection nebulae. Further analysis of these objects will reveal the energy injection source of the Galactic center diffuse X-ray emission.