Indirect X‐Ray Line‐Formation Processes in Iron L‐Shell Ions

Abstract

We present a systematic study of the role of indirect processes in the soft X-ray line formation of iron L-shell ions, using a newly developed, relativistic, multiconfigurational atomic package. These indirect processes involve the neighboring charge states of the target ion, namely, radiative recombination, dielectronic recombination, and resonance excitation. For Fe XVII, the inner-shell collisional ionization (CI) is also relevant. Lines originating from 3s and 3p upper levels of Fe XVII-XX are found to be significantly affected by these processes, with some lines being enhanced by nearly a factor of 2 at the temperature of maximum fractional abundance in CI equilibrium. Such enhancement, although not enough to completely explain the observed line ratios from various astrophysical sources, is a vast improvement over the previous models neglecting these processes. Rate coefficients for individual processes are tabulated, which can be conveniently included in spectral models to correctly account for these effects. As a by-product of this investigation, the total recombination and ionization rates of all Fe L-shell ions are tabulated as a function of temperature.