X-ray spectroscopy in the microcalorimeter era II: A new Diagnostic on Column Density from the Case A to B Transition in H- and He-like Iron
Abstract
The Soft X-ray Spectrometer (SXS) on board Hitomi, with the unprecedented resolving power of R1250, allowed the detection of members of the Fe XXV \ 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- and two-electron systems. We compare the Fe XXV Kα 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 datasets, and obtain good agreement. We also show the effect of turbulence on Fe XXV Kα line ratios and interplay between column density and metallicity. Besides, we discuss the atomic number dependence of transition probabilities for allowed and non-allowed transitions, which causes the highly charged He-like systems, such as Fe XXV, to behave fundamentally differently from He I.