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Register a new userHigh-Resolution X-ray Spectroscopy of the Interstellar Medium II: Neon and Iron Absorption Edges
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Adrienne M. Juett
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We present high-resolution spectroscopy of the neon K-shell and iron L-shell interstellar absorption edges in nine X-ray binaries using the High Energy Transmission Grating Spectrometer (HETGS) onboard the Chandra X-ray Observatory. We found that the iron absorption is well fit by an experimental determination of the cross-section for metallic iron, although with a slight wavelength shift of ~20 mA. The neon edge region is best fit by a model that includes the neutral neon edge and three Gaussian absorption lines. We identify these lines as due to the 1s-2p transitions from Ne II, Ne III, and Ne IX. As we found in our oxygen edge study, the theoretical predictions for neutral and low-ionization lines all require shifts of ~20 mA to match our data. Combined with our earlier oxygen edge study, we find that a best fit O/Ne ratio of 5.4+/-1.6, consistent with standard interstellar abundances. Our best fit Fe/Ne ratio of 0.20+/-0.03 is significantly lower than the interstellar value. We attribute this difference to iron depletion into dust grains in the interstellar medium. We make the first measurement of the neon ionization fraction in the ISM. We find Ne II/Ne I ~ 0.3 and Ne III/Ne I ~ 0.07. These values are larger than is expected given the measured ionization of interstellar helium. For Ne IX, our results confirm the detection of the hot ionized interstellar medium of the Galaxy.
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(Abbrev.) We present high-resolution spectroscopy of the oxygen K-shell interstellar absorption edge in 7 X-ray binaries using the HETGS onboard Chandra. Using the brightest sources as templates, we found a best-fit model of 2 absorption edges and 5 Gaussian absorption lines. All of these features can be explained by the recent predictions of K-shell absorption from neutral and ionized atomic oxygen. We identify the K alpha and K beta absorption lines from neutral oxygen, as well as the S=3/2 absorption edge. The expected S=1/2 edge is not detected in these data due to overlap with instrumental features. We also identify the K alpha absorption lines from singly and doubly ionized oxygen. The OI K alpha absorption line is used as a benchmark with which to adjust the absolute wavelength scale for theoretical predictions of the absorption cross-sections. We find that shifts of 30-50 mA are required, consistent with differences previously noticed from comparisons of the theory with laboratory measurements. Significant oxygen features from dust or molecular components, as suggested in previous studies, are not required by our HETGS spectra. With these spectra, we can begin to measure the large-scale properties of the ISM. We place a limit on the velocity dispersion of the neutral lines of <200 km s^{-1}, consistent with measurements at other wavelengths. We also make the first measurement of the oxygen ionization fractions in the ISM. We constrain the interstellar ratio of OII/OI to ~0.1 and the ratio of OIII/OI to <0.1.
We find resolved interstellar O K, Ne K, and Fe L absorption spectra in the Chandra Low Energy Transmission Grating Spectrometer spectrum of the low mass X-ray binary X0614+091. We measure the column densities in O and Ne, and find direct spectroscopic constraints on the chemical state of the interstellar O. These measurements probably probe a low-density line of sight through the Galaxy and we discuss the results in the context of our knowledge of the properties of interstellar matter in regions between the spiral arms.
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.
We present a survey of six low to moderate redshift quasars with Chandra and XMM-Newton. The primary goal is to search for the narrow X-ray absorption lines produced by highly ionized metals in the Warm-Hot Intergalactic Medium. All the X-ray spectra can be fitted by a power law with neutral hydrogen absorption method. The residuals that may caused by additional emission mechanisms or calibration uncertainties are taken account by polynomial in order to search for narrow absorption features. No real absorption line is detected at above 3-sigma level in all the spectra. We discuss the implications of the lack of absorption lines for our understanding of the baryon content of the universe and metallicity of the intergalactic medium (IGM). We find that the non-detection of X-ray absorption lines indicates that the metal abundance of the IGM should be smaller than ~0.3 solar abundance. We also discuss implications of the non-detection of any local (z ~ 0) X-ray absorption associated with the ISM, Galactic halo or local group, such as has been seen along several other lines of sight (LOS). By comparing a pair of LOSs we estimate a lower limit on the hydrogen number density for the (z ~ 0) 3C 273 absorber of n_H >= 4e-3 cm^-3.
We present an X-ray absorption model for the interstellar medium, to be referred to as ISMabs, that takes into account both neutral and ionized species of cosmically abundant elements, and includes the most accurate atomic data available. Using high-resolution spectra from eight X-ray binaries obtained with the Chandra High Energy Transmission Grating Spectrometer, we proceed to benchmark the atomic data in the model particularly in the neon K-edge region. Compared with previous photoabsorption models, which solely rely on neutral species, the inclusion of ions leads to improved spectral fits. Fit parameters comprise the column densities of abundant contributors that allow direct estimates of the ionization states. ISMabs is provided in the appropriate format to be implemented in widely used X-ray spectral fitting packages such as XSPEC, ISIS and SHERPA.
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