X-ray line diagnostics of the multi-phase gas in the Centaurus cluster core with XRISM/Resolve

TL;DR

This study uses XRISM/Resolve's high-resolution X-ray spectroscopy to analyze the multi-phase, biphasic temperature structure of the intracluster medium in the Centaurus cluster core, revealing detailed ionization states and evidence of resonant scattering.

Contribution

First high-resolution X-ray spectral analysis of the Centaurus cluster core revealing a biphasic temperature structure and ionization diagnostics with XRISM/Resolve.

Findings

The ICM is well-described by a two-temperature plasma at ~1.6 and ~3 keV.

Line ratios indicate a trend of increasing ionization temperature with atomic mass.

Resonant scattering effects are suggested by flux ratio reductions in Fe lines.

Abstract

We report the multi-temperature structure of the intracluster medium (ICM) in the Centaurus cluster core observed with XRISM/Resolve. Thanks to its high energy resolution, Resolve enables us to measure fine structures of highly ionized emission lines from Si to Fe and to directly determine the excitation temperature and the ionization temperature from the emission line ratio diagnostics. The observed spectrum in the Centaurus core is well-represented by a double-temperature thermal plasma at collisional ionization equilibrium state rather than an isothermal one. The line ratio diagnostics also support this biphasic temperature structure. Particularly, the observed line ratios show a trend of increasing ionization temperature with atomic mass, while the ionization and excitation temperatures of Fe show nearly the same temperature. The resultant line ratios, which are well-represented by the two temperatures ICM, ~ 1.6 and ~ 3 keV, are also fairly consistent with the expected numbers when assuming the radial single-temperature ICM was projected in the cluster core along the line of sight. Due to the limited low-energy sensitivity of the Resolve with the gate valve closed, we investigated the effect of the cool component using the XMM-Newton/RGS spectrum, but it ultimately did not affect our results. The observed flux ratio between the Fe XXV He alpha resonance and forbidden lines shows an about 20% reduction, suggesting the presence of resonant scattering.