Energy shift of Fe-K fluorescence lines due to low ionization demonstrated with XRISM in Centaurus X-3

TL;DR

This study demonstrates that the ionization degree of iron affects the energy of Fe K$eta$ and K$eta$ lines, and uses XRISM data of Cen X-3 to measure this effect, providing a new method to estimate ionization states in astrophysical environments.

Contribution

The paper introduces a novel approach to determine iron ionization states using differential line shifts in high-resolution X-ray spectra, validated with XRISM observations of Cen X-3.

Findings

Fe K$eta$ line shifts blueward by ~30 eV from neutral to Fe$^{8+}$.

The differential shift between K$eta$ and K$eta$ lines estimates ionization degree.

Ionization correction aligns systemic velocity measurements with optical data.

Abstract

The Fe K$\alpha$ fluorescence line at 6.4 keV is a powerful probe of cold matter surrounding X-ray sources and has been widely used in various astrophysical contexts. The X-ray microcalorimeter spectrometer onboard XRISM can measure line shifts with unprecedented precision of $\sim$0.2 eV, equivalent to a line-of-sight velocity of $\sim$10 km s$^{-1}$. At this level of accuracy, however, several factors that influence the line energy must be carefully considered prior to astrophysical interpretation. One such important factor is the ionization degree, Fe$^{q+}$. The K$\alpha$ line shifts redward by $\sim$4 eV as $q$ increases from 0 (neutral) to 8 (Ar-like). Additionally, the accompanying Fe K$\beta$ line at 7.06 keV shifts blueward by $\sim$30 eV from $q=0$ to 8. We demonstrate that this effect is actually observable in the XRISM data of the high-mass X-ray binary Centaurus X-3 (Cen X-3). We advocate that the differential energy shift between the K$\alpha$ and K$\beta$ line provides a robust estimate of $q$ by decoupling from other effects that shift the two lines in the same direction. We derived $q \sim 5$ (Sc-like) for the fluorescing matter by comparing the observation with atomic structure calculations of our own and in the literature. By accounting for the derived charge state and the corresponding shift in the rest-frame line energy, we made corrections for this effect and reached a consistent residual shift among the K$\alpha$, K$\beta$, and the optical measurement attributable to the systemic velocity of the system. Consequently, we obtained a new constraint on the location of the cold matter. This ionization effect needs to be assessed in all use cases of the Fe K$\alpha$ line shift beyond Cen X-3, and the proposed metric is generally applicable to all of them.