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

Marie Kondo, Kotaro Fukushima, Kazunori Suda, Anwesh Majumder, Kosuke Sato, Kyoko Matsushita, François Mernier, Kazuhiro Nakazawa, Aurora Simionescu, Jean-Paul Breuer, Yasushi Fukazawa, Ryuichi Fujimoto, Isamu Hatsukade, Kokoro Hosogi, Michael Loewenstein, Tom'aš Plšek, Ming Sun, Misaki Urata, Norbert Werner, Noriko Y. Yamasaki, Yutaka Fujita

TL;DR

The study addresses the temperature structure of the Centaurus cluster core by leveraging XRISM/Resolve’s high-resolution X-ray spectroscopy to perform line-diagnostics across Si–Fe, enabling direct measurements of excitation and ionization temperatures. The analysis reveals a biphasic ICM best described by two temperatures, approximately 1.6 keV and 3 keV, rather than a single isothermal component, with line ratios largely consistent with these two phases and with projection effects along the line of sight. Notably, the ionization temperature tends to increase with atomic mass, while Fe’s ionization and excitation temperatures are nearly equal, and the Fe XXV Heα resonance-to-forbidden line flux ratio shows about a 20% reduction indicative of resonant scattering. XMM-Newton/RGS data are used to probe a potential cool component, but it does not alter the main XRISM/Resolve results, reinforcing the robustness of the two-temperature interpretation for the core’s multi-phase gas. These findings demonstrate the power of high-resolution line diagnostics for unraveling the complex thermal structure of cool-core clusters and provide empirical constraints on gas heating and mixing processes in the ICM.

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.

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

TL;DR

The study addresses the temperature structure of the Centaurus cluster core by leveraging XRISM/Resolve’s high-resolution X-ray spectroscopy to perform line-diagnostics across Si–Fe, enabling direct measurements of excitation and ionization temperatures. The analysis reveals a biphasic ICM best described by two temperatures, approximately 1.6 keV and 3 keV, rather than a single isothermal component, with line ratios largely consistent with these two phases and with projection effects along the line of sight. Notably, the ionization temperature tends to increase with atomic mass, while Fe’s ionization and excitation temperatures are nearly equal, and the Fe XXV Heα resonance-to-forbidden line flux ratio shows about a 20% reduction indicative of resonant scattering. XMM-Newton/RGS data are used to probe a potential cool component, but it does not alter the main XRISM/Resolve results, reinforcing the robustness of the two-temperature interpretation for the core’s multi-phase gas. These findings demonstrate the power of high-resolution line diagnostics for unraveling the complex thermal structure of cool-core clusters and provide empirical constraints on gas heating and mixing processes in the ICM.

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.
Paper Structure (4 sections, 2 figures, 2 tables)

This paper contains 4 sections, 2 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observation and data reduction
  3. XRISM/Resolve
  4. XMM-Newton/RGS

Figures (2)

  • Figure 1: The XRISM/Resolve FoV (green box) overlaid on the Chandra X-ray image in the 2.0--8.0 keV energy band for the Centaurus cluster core. The missing part of the FoV corresponds to the calibration pixel 12. The extraction region for the XMM-Newton/RGS spectral analysis is shown by white solid lines. The data sets are available at https://doi.org/10.25574/cdc.500. Alt text: One X-ray image.
  • Figure 2: Upper left panel shows the zoom-in spectra around Fe He$\alpha$ triplet for the Resolve entire region spectrum. The orange, red, blue, and light blue solid lines indicate the best-fit models: (a) "1T", (b) "1T w/ zgauss", (c) "2T w/ zgauss", and (d) "gadem w/ zgauss", respectively. The upper right panels show the residuals corresponding to models (a) to (d). The gray-shaded region shows the energy range around the Fe .8 .8 XXV resonance line. The lower panels show the zoom-in Resolve spectral fits in the 2.2--2.7 keV band (left) and the 6.7--9.0 keV band (right). The thin blue solid and dashed lines indicate the higher and lower temperature components of the "2T w/ zgauss" model, respectively. The middle panels show the residuals between the data and the "1T w/ zgauss" and "2T w/ zgauss" models. The bottom panels also show the residuals between the data and the "1T w/ zgauss" and "gadem w/ zgauss" models. Alt text: Seven line graphs. The upper left graph shows X-ray spectra near the He $\alpha$ Fe line. The upper right plots display residuals for four test models. In the two plots shown below, the y-axis of the upper panel represents the count rate per kiloelectron volt, whereas the y-axes of the middle and lower panels show the residuals between the data and the model.