Iron and Nickel Line Diagnostics for the Galactic Center Diffuse Emission

Katsuji Koyama(1), Yoshiaki Hyodo(1), Tatsuya Inui(1), Hiroshi Nakajima(1), Hironori Matsumoto(1), Takeshi Go Tsuru(1), Tadayuki Takahashi(2), Yoshitomo Maeda(2), Noriko Yamazaki(2), Hiroshi Murakami(3), Shigeo Yamauchi(4), Yohko Tsuboi(5), Atsushi Senda(6), Jun Kataoka(7), Hiromitsu Takahashi(8), Stephen S Holt(9), Gregory V Brown(10)

(1) Department of Physics, Graduate school of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502
(2) Institute of Space and Astronautical Science, JAXA, Sagamihara, Kanagawa, 229-8510
(3) PLAIN center, ISAS/JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510
(4) Faculty of Humanities and Social Sciences, Iwate University, 3-18-34 Ueda, Morioka, Iwate 020-8550
(5) Department of Physics, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga,, Bunkyo-ku, Tokyo 112-8551, Japan
(6) The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198
(7) Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo, 152-8551
(8) Department of Physical Science, School of Science, Hiroshima University, 1-3-1 Kagamiyama,, Higashi-Hiroshima, Hiroshima 739-8526
(9) Olin College, Needham, MA 02492 USA (10) High Energy Density Physics and Astrophysics Division, Lawrence Livermore National Laboratory,, Livermore, California 94551, USA

Paper: PASJ, accepted

EPrint Server: astro-ph/0609215


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 provide 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 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 K alpha ) and hydrogenic (Fe XXVI Ly alpha ) ions of iron. In addition, K alpha lines from neutral or low ionization nickel (Ni I K alpha ) and He-like nickel (Ni XXVII K alpha ), and Fe I K beta , Fe XXV K beta , Fe XXVI Ly beta , Fe XXV K gamma and Fe XXVI Ly gamma are detected for the first time. The line center energies and widths of Fe XXV K alpha and Fe XXVI Ly alpha favor a collisional excitation (CE) plasma for the origin of the GCDX. The electron temperature determined from the line flux ratio of Fe XXV-K alpha / Fe XXV-K beta is similar to the ionization temperature determined from that of Fe XXV-K alpha /Fe XXVI-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.

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