K-Shell Photoabsorption in Si11+ Including Relativistic Effects

As part of a complete investigation into the entire Si isonuclear sequence for x-ray spectral diagnostics, we focus on the Li-like Si11+ ion, which is a simpler 3-electron system, is susceptible to stronger relativistic effects due to the large charge, and also features in x-ray astrophysical spectral models.  R-matrix calculations are carried out for the photoabsorption cross section of Si11+ including 1) the Auger broadening due to spectator Auger decay of K-shell excited autoionizing resonances, and 2) relativistic contributions via use of a Breit-Pauli R-matrix (BPRM) approach. Computed background cross sections are in good overall agreement with earlier resonanceless photoionization cross sections, indicating that the direct  photoabsorption features are taken into account properly.  Comparison with earlier BPRM cross sections  [M.~C.~Witthoeft, et al., ApJ 192, 7 (2011)] shows good agreement in general but the new calculations now resolve earlier step-like discontinuities and improve relaxation energies.  The computed cross sections are important for the detailed analysis of the gaseous component of the atomic and ionic Si K edge by using high  resolution Chandra spectra of low-mass X-ray binaries.