X-Ray Absorption by Interstellar Atomic Gases near the K Edges of C, O, Ne, Mg, and Si and the L Edge of Fe

As part of an ongoing research program, we have calculated accurate atomic photoabsorption cross sections using robust numerical methods such as the R-Matrix, multiconfiguration Hartree-Fock (MCHF), and AUTOSTRUCTURE computer packages. They are further optimized to treat additional relevant inner-shell effects, e.g., Auger and radiative damping, orbital relaxation effects, and pseudorbitals. To date, neutral and ionized C, O, Ne, and Mg have been studied in detail, combining R-Matrix and MCHF analyses with laboratory measurements and X-ray observations. The final calibrated atomic data provide reliable absorption models for interpreting interstellar X-ray absorption spectra. Our current X-ray absorption studies focus on the heavier, more complicated third- and fourth-row atomic systems such as the Si K-edge region (≈1.85 keV), where the photoabsorption cross section is needed to understand an observed near-threshold Chandra spectrum. Atomic Fe is also an important system with L-edge photoabsorption peaking near the fine-structure 2p^{-1 2}P_3/2,1/2 split thresholds at ≈710-720 eV, which require a relativistic treatment.