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We study the composition of the Galactic interstellar medium (ISM) toward the Galactic center region (5 < |l| < 20 degree) by utilizing X-ray absorption features of three bright low-mass X-ray binaries (LMXBs), GX 13+1, GX 5-1, and GX 340+0, observed with the Chandra HETGS. We detect X-ray absorption fine structure (XAFS) of the Si K-edge, characterized by a narrow and a broad absorption feature at 1846 and ~1865 eV, respectively. Comparison with ground experimental data indicates that most of the ISM Si exists in the form of silicates, although a composition of "pure" forsterite is ruled out. The XAFS spectra of the sulfur K-edge indicate that a significant fraction of S exists in the gas phase. From each source, we derive the column densities of Mg, S, Si, and Fe from the K-edge depth and that of O (or H) from the absorption of the continuum. The elemental abundance ratios are found to be consistent between the three targets: the mean values of O/Si, Mg/Si, S/Si, and Fe/Si are determined to be 0.55+-0.17, 1.14+-0.13, 1.03+-0.12, and 0.97+-0.31 solar, respectively (90% error in the mean value). We discuss the origins of the overabundances of the heavy metals relative to O in the Galactic ISM by comparison with the abundance pattern of the intracluster medium in clusters of galaxies. Assuming that most of the Mg and Si atoms are depleted into silicates of either the proxine or olivine family, we estimate that the number ratio of Mg to Fe in olivine is >~1.2 and that 17%-43% of the total O atoms in the ISM must be contained in silicate grains., Comment: 31 pages, 15 figures, accepted for publication in ApJ, vol. 620, 2005. Proof corrections are reflected (column densities of O and H were overestimated in the previous version.). Figures 1(a)-(c) are revised

This paper reports the diffuse X-ray features around the Galactic center observed with Chandra. We confirm the ASCA and Ginga discoveries of the large-scale thin-thermal plasma with strong lines in the Galactic center region. In addition, many small clumps of emission lines from neutral (6.4 keV line) to He-like (6.7 keV line) irons are discovered. The 6.4 keV line clumps would be reflection nebulae, while those of the 6.7 keV line are likely SNRs. We also find emission lines of intermediate energy between 6.5-6.7 keV, which are attributable to young SNRs in non equilibrium ionization. Non-thermal filaments or belts with X-ray spectra of no emission line are found, suggesting the Fermi acceleration site in a rapidly expanding shell. All these suggest that multiple-supernovae or extremely large explosion had occurred around the Galactic center region in the recent past., Comment: 4 pages, 4 figures, Proceedings of the Symposium "New Visions of the X-ray Universe in the XMM-Newton and Chandra era", Noordwijk-NL, November 2001 for ps-file with high resolution figures, see http://www-cr.scphys.kyoto-u.ac.jp/member/bamba/Proceeding/Newvision.ps.gz

We report the ASCA and Chandra discovery of a diffuse X-ray source in the Galactic center region. The X-ray spectrum is fitted with a non-equilibrium ionization (NEI) plasma model of about 6-keV temperature. The model requires higher than solar metal abundances, a young plasma age of $\simeq$ 100 years and a large N_H value of about 10^{23} cm^{-2}. The N_H value constrains the source position to be in the Galactic center region at about 8.5 kpc distance. The high resolution X-ray image with the Chandra ACIS shows a ring of 10'' radius which corresponds to 0.4 pc at the Galactic center, and a tail-like structure. Although the morphology is peculiar, the other X-ray features are likely to be a very young supernova remnant, possibly in a free expansion phase., Comment: 11 pages, 4 figures, to be published in ApJ

The ASCA satellite made 107 pointing observations on a 5 x 5 deg^2 region around the center of our Milky Way Galaxy (the Galactic Center) from 1993 to 1999. In the X-ray images of the 0.7--3 keV or 3--10 keV bands, we found 52 point sources and a dozen diffuse sources. All the point sources are uniformly fitted with an absorbed power-law model. For selected bright sources, Sgr A*, AX J1745.6-2901, A 1742-294, SLX 1744-300, GRO J1744-28, SLX 1737-282, GRS 1734-292, AX J1749.2-2725, KS 1741-293, GRS 1741.9-2853, and an unusual flare source XTE J1739-302, we present further detailed spectral and timing analyses, and discuss their nature. The dozen extended X-ray sources comprise radio supernova remnants, giant molecular clouds, and some new discoveries. Most show emission lines from either highly ionized atoms or low-ionized irons. The X-ray spectra were fitted with either a thin thermal or power-law model. This paper summarizes the results and provides the ASCA X-ray source catalogue in the Galactic Center region., Comment: 33 pages, 8 figures, Accepted for publication in ApJS, also found in http://www.star.le.ac.uk/~mas/research/paper/#Sakano2001apjs

From the ASCA X-ray point-source list in the Galactic center 5x5 degree^2 region, we found the clear correlation between the position of the sources and the absorption. This fact implies that the major part of the absorption is due to the cold interstellar matter (ISM) in the line of sight. Using the correlation, we estimate the distribution of the cold ISM. We also found that the ratio of high mass binaries to low mass binaries is significantly smaller than that in the whole Galaxy or SMC, which implies that the past starburst activity in the Galactic center region was rather quiet., Comment: submitted to Proceeding of "New Century of X-ray Astronomy", 6-8 March 2001, Yokohama, Japan (2 pages, 1 Fig, requires pasp.sty), also found in http://www-maxi.tksc.nasda.go.jp/%7Esakano/work/paper/ index-e.html#Sakano2001asca2001