X-Ray Spectroscopy in the Microcalorimeter Era. II. A New Diagnostic on Column Density from the Case A to B Transition in H- A nd He-like Iron

The Soft X-ray Spectrometer on board Hitomi, with the unprecedented resolving power of R ∼ 1250, allowed the detection of members of the Fe xxv Kα complex emission spectra from the center of the Perseus Cluster. In this paper, we introduce a novel method of measuring the column density using the optically thin (Case A) to optically thick (Case B) transition for one- A nd two-electron systems. We compare the Fe xxv K α line ratios computed with CLOUDY with that from the Hitomi observations in the outer region of the Perseus core using collision strengths from different atomic data sets, and obtain good agreement. We also show the effect of turbulence on Fe xxv Kα line ratios and interplay between column density and metallicity. Additionally, we discuss the atomic number dependence of transition probabilities for allowed and unallowed transitions, which causes highly charged He-like systems, such as Fe xxv, to behave fundamentally differently from He i.