Coulomb-collision-mediated temperature equilibration between electrons, protons, and He-like iron ions (Fe XXV) in the limit of adiabatic heating of electrons at the shock. The Fe XXV ions temperature, T_Z, approaches T_p at τ ∼ 10¹² cm⁻³ s and then evolves together with T_p. The NEI timescale (see Fig. 2 below) is approximately the same as the T_Z ≫ T_p timescale. Ions remain hotter than electrons up to τ ∼ 2 − 3 × 10¹³ cm⁻³ s. Therefore, the Fe XXV ions are hotter than T_e up to this value of τ, and the thermal broadening of the Fe XXV lines is larger than that predicted from the electron temperature T_e derived from the continuum shape. The helium abundance is set to zero for this plot (see Fig. C.1 for the impact of helium).