Sound speed normalized to the kick velocity c_s/V_k (left panel) and relativistic Mach number ℳ (right panel) at t = 99.46h for model S.00 when subject to a recoil of V_k = 300 km s^-1. Note that V_k ≥ c_s is not a good criterion for the localization of the shock (cf. left panel) and that no obvious correlation is present between the supersonicity of the flow and the appearance of the shock (cf. right panel).