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R.A. Benjamin (U. Wisconsin)
Calculations of opacity of the interstellar medium as a function of photon energy from 13.6 eV to 10 keV are presented. The mean absorption cross section is calculated for molecular, neutral, and warm ionized gas, and compared to previous calculations. Noteworthy points include the following: (1) Revision in the reference interstellar metal abundances to 0.2 dex below the solar value, motivated by several lines of evidence, decreases the opacity of the ISM at energies above 0.55 keV, (2) the ionization edges of neutral species are shifted by several eV from previous calculations, and (3) absorption by ionized atoms and molecular hydrogen have been included for the first time. Analytical fits accurate to a few percent for the mean absorption cross sections are provided.
The potential use of X-ray absorption edges to diagnose the ionization and chemical state of atoms is also discussed, concentrating on the K-shell edges of carbon and oxygen. I characterize the shifts of the absorption edges with ionization state, the expected X-ray absorption fine structure that occurs when these atoms are in the solid phase, and the dramatic changes in absorption cross section shapes when these atoms are incorporated into molecules. For CO, the presence of \sigma* resonances increases the carbon near-edge absorption by up to 60% and the oxygen near-edge cross section by 35% over a 20 eV range, as well as producing detectable shifts in the \pi* resonances. Most of these details of near-edge structure should be detectable with AXAF and future missions.
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