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Astrophysics Research and Analysis Program

Accurate atomic data for analyzing high-resolution X-ray absorption spectra

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Accurate atomic data for analyzing high-resolution X-ray absorption spectra
Since the launch of Chandra and XMM Newton, there has been great interest in high-resolution X-ray spectroscopy of a variety of sources. While the early high-resolution observations focused on emission lines from stellar coronae, X-ray binaries, and AGNs, the focus has shifted to the study of absorption features of X-ray binaries and AGNS as well as of the interstellar medium. X-ray absorption features produced by ionization edges and resonance lines reveal rich information about the absorbing material, such as column densities, outflow or inflow velocities and intrinsic velocity broadening. While many of the absorption lines are due to the well known transitions in H-like and He-like ions, the more complex inner-shell resonance transitions in lower ionization species have been clearly detected in many sources. This detection includes K-shell transitions of various L-shell ions in numerous AGN absorption spectra. Moreover, L-shell transitions of M-shell iron, the so-called Fe M-shell unresolved transition array (UTA) has become an important diagnostic tool for low ionization material in AGN outflows. Although there has been extensive work in the area of theoretical and laboratory atomic physics to improve the data used in plasma codes in recent years, the accuracy and completeness of available data for inner-shell absorption are not yet satisfactory. In particular, the wavelengths of inner-shell transitions for the majority of ions are not accurate enough for reliable identification of complex absorption features. In cases, where the proper identification could be made, the available theoretical wavelength data are typically of insufficient accuracy for reliable kinematic measurements through the Doppler shifts. We propose to provide vastly improved theoretical energy level and transition energy data for the K-shell transitions of the L-shell ions of C, N, O, Ne, Mg, Si, S, and Ar as well as for the L-shell transitions of the M-shell ions of Fe. Our calculations will be made with the multi-reference Møller-Plesset atomic physics code, which has recently been shown to provide transition wavelengths with spectroscopic accuracy for the M-shell transitions in Fe XVI and Fe XV L-shell ions. The proposed calculations will, thus, extend our L-shell transition calculations to lower iron charge states and will produce K-shell transitions of astrophysically important L-shell ions with similar spectroscopic accuracy. In all cases, we will optimize our calculations to ensure that they reproduce the (few) available wavelengths from laboratory sources. More »

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