We study the composition of the Galactic interstellar medium ( ISM ) toward the Galactic center region ( 5 ^ { \circ } < |l| < 20 ^ { \circ } ) 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 eV and \approx 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 \pm 0.17 , 1.14 \pm 0.13 , 1.03 \pm 0.12 , and 0.97 \pm 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 \buildrel > \over { \sim } 1.2 and that 17 % –43 % of the total O atoms in the ISM must be contained in silicate grains .