X-ray line ratio 3C/3D in Fe XVII

Abstract

The X-ray line intensity ratio 3C/3D in Fe XVII is potentially one of the most useful plasma diagnostic line ratios for X-ray astronomy. Based on our recent converged and highly accurate atomic calculations for electron impact excitation of Fe XVII using the fully relativistic close-coupling method, we demonstrate that 3C/3D in Fe XVII is not a constant with respect to electron temperature/energy. The cascade effects (including the excitation and recombination population pumping) on 3C/3D as a function of temperature (for Maxwellian plasmas) or electron beam energy (for some laboratory plasmas) are studied and elaborated. For Maxwellian plasmas prevailing in astrophysical sources under the condition of collisional ionization equilibrium, a new and highly accurate 3C/3D line ratio is reported. The values of 3C/3D reported in this work are 2.66, 2.76, 2.81, 2.88 and 2.94 for electron temperatures of 2, 3, 4, 6 and 10 MK, respectively. The uncertainty in our new line ratio is estimated to be about 5 per cent. These new 3C/3D values are expected to have immediate applications for a wide range of X-ray spectra observed from both point sources as well as extended sources. © 2008 The Author. Journal compilation © 2008 RAS.