Dirac R-matrix calculation for electron-impact excitation of S xiii

Context. Sulfur emission lines in the soft X-ray and extreme-ultraviolet regions are observed in a variety of laboratory and astrophysical spectra. But accurate electron impact excitation data for S xiii for state-of-the-art NLTE spectral models are scarce.
Aims: We calculated electron-impact excitation collision strengths and effective collision strengths of S xiii for transitions among the lowest-lying 98 fine-structure states 1s²2lnl' corresponding to principal quantum numbers n = 2,3,4. The effective collision strengths for these transitions were computed over a wide temperature range (log ₁₀T_e (K) = 4.53-7.53) for various astrophysical plasma conditions.
Methods: We used the fully-relativistic parallel Dirac R-matrix code to calculate collision strengths. To generate target wavefunctions and energy levels for scattering calculations, we employed the GRASP0 multi-configuration Dirac-Fock code for states up to n = 5.
Results: The wavefunctions are generated from 27 configurations - 1s²2lnl'(n = 2,3,4,5) - giving rise to 166 jj energy levels. The collision and effective collision strengths among the lowest 98 fine-structure levels are compared with the previous theoretical calculations. The collision strengths for most transitions agree well at higher incident electron energies.
Conclusions: The resonant contributions to effective collision strengths are most dominant at lower temperatures.