Electron–Ion Equilibration in the Merging Galaxy Cluster A665

Abstract Galaxy cluster mergers drive powerful shock fronts that heat the intracluster medium (ICM) and accelerate particles, redistributing the energy in a merger. A665 is one of only a few clusters with such a powerful shock ( M ∼ 3), and it provides a unique opportunity to study the thermalization timescale of the ICM, particularly the electron–ion equilibration timescale. Understanding this timescale is crucial for determining how the energy from the merger is distributed between thermal and nonthermal particle populations. Using ∼200 ks of NuSTAR observations, we measure the temperature distribution across the shock to distinguish between two heating models: (1) an instant collisionless model, where ions and electrons are immediately heated at the shock front; and (2) a collisional model, where electrons are initially adiabatically compressed at the shock and subsequently equilibrate with the ions over ∼100 Myr. Our measurements favor the delayed-equilibration model, suggesting that electrons do not immediately reach thermal equilibrium with the ions at the shock front and instead equilibrate over t eq = (4.0 ± 3.4) × 10 8 yr. Additionally, our temperature measurements indicate that the Mach number may be lower than previously estimated ( M = 2 . 8 ± 0 . 7 ), suggesting that the shock strength has been overestimated in past studies. These results add to our understanding of the microphysics governing how thermal energy is distributed in diffuse plasmas like the ICM, with implications for galaxy cluster evolution, large-scale structure formation, and cosmology.