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

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 Ti > Te 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.

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