Iron He-triplet signatures of shocks in the hottest galaxy clusters

Z/W line ratio, line broadening, and electron-ion temperature equilibration

¹ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, D-85741 Garching, Germany
² Department of Astronomy, University of Maryland, College Park, MD 20742, USA
³ X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴ Center for Research and Exploration in Space Science and Technology, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Rudolf Peierls Centre for Theoretical Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Rd, Oxford OX1 3PU, UK
⁶ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
⁷ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
⁹ Department of Theoretical Physics and Astrophysics, Masaryk University, Brno 61137, Czechia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 April 2026
Accepted: 8 June 2026

Abstract

A merger of clusters naturally drives shocks with Mach number ℳ ≲ 3 in the intracluster medium (ICM). This process creates several distinct signatures, including sharp surface-brightness “edges”, temperature, and gas-velocity jumps. The low density of the ICM implies that ionization balance and electron-ion equilibration timescales can be long enough to produce additional observable signatures. Here, we focus on two “transient” spectral signatures accessible with high-energy-resolution telescopes such as XRISM, even in unfavorable geometries, for example when we are looking inside the Mach cone of the shock, which precludes the appearance of sharp edges in X-ray images. In this work, we focus on (i) the Z/W line ratio of the Fe XXV triplet and (ii) the contribution of ions with T_i > T_e to the line width, which might be mistakenly interpreted as gas turbulence. We demonstrate that the Z/W ratio can serve as a proxy for the nonequilibrium state of the shocked ICM and facilitate interpretation of the line broadening. We conclude that these spectral signatures are within reach of missions such as XRISM and can be used to constrain the electron heating at collisionless cluster shocks, as well as the rate of subsequent temperature equilibration between different particle species.