Electron distributions in X-Ray plasmas: spectral diagnostics with the 3s/3d line ratio in Fe XVII

Abstract

Efforts to benchmark astrophysical observations with X-ray laboratory measurements have been stymied by observed and measured differences of up to a factor of two in the ratio '3s/3d' of Fe XVII lines at ~17 \AA and ~15 \AA respectively. Using the electron distribution function (EDF) as a new physical parameter, we compute the Fe XVII line ratios and account for these differences. Based on large-scale relativistic close coupling calculations using the Breit-Pauli R-matrix method, revealing the precise effect of resonances in collisional excitation, we employ collisional-radiative models using cross sections convolved with both the Gaussian and the Maxwellian EDF. Comparison with astrophysical observations and laboratory measurements demonstrates that (a) the 3s/3d line ratio depends not only on the EDF but also on the electron temperature/energy of the source, (b) plasma conditions in experimental measurements and astrophysical observations may be quite different, and (c) departure from a Maxwellian should manifest itself in, and be used as a diagnostics of, particle distributions in plasmas.