Iron He-triplet signatures of shocks in the hottest galaxy clusters. Z/W line ratio, line broadening, and electron-ion temperature equilibration

Abstract

A merger of clusters naturally drives shocks with Mach number $\mathscr{M}\lesssim 3$ in the intra-cluster 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 the ionization balance and electron-ion equilibration times can be long enough to produce a set of additional observable signatures. Here, we focus on two "transient" spectral signatures accessible with the high-energy-resolution telescopes such as XRISM, even for unfavorable geometry, e.g., when we are looking inside the Mach cone of the shock, precluding the appearance of sharp edges in X-ray images. In this work, we focus on (i) the $\mathtt{Z/W}$ line ratio of the Fe~XXV triplet and (ii) the contribution of ions with $T_i>T_{\rm e}$ to the line width, which might be mistakenly interpreted as the gas turbulence. We demonstrate that the $\mathtt{Z/W}$ ratio can serve as a proxy for the non-equilibrium state of the shocked ICM and facilitate interpretation of the line broadening. We conclude that these spectral signatures are within reach with missions like XRISM and can be used to constrain the heating of electrons at the collisionless cluster shocks, as well as the rate of subsequent temperature equilibration between different particle species.