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

TL;DR

This paper introduces a new method for measuring column density in X-ray spectra using the transition from optically thin to thick regimes, demonstrated with Hitomi observations of the Perseus Cluster.

Contribution

It presents a novel diagnostic technique based on Case A to B transitions in Fe XXV, validated with observational data and atomic data comparisons.

Findings

Good agreement between CLOUDY models and Hitomi data.

Turbulence affects Fe XXV line ratios significantly.

Atomic number influences transition probabilities in highly charged ions.

Abstract

The Soft X-ray Spectrometer (SXS) on board \textit{Hitomi}, with the unprecedented resolving power of R$\sim$1250, allowed the detection of members of the Fe XXV \ka\ complex emission spectra from the center of the Perseus Cluster. In this paper, we introduce a novel method of measuring the column density using the optically thin (Case A) to optically thick (Case B) transition for one- and two-electron systems. We compare the Fe XXV K$\alpha$ line ratios computed with CLOUDY with that from the \textit{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 K$\alpha$ 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.