The precursor structure in relativistic shocks

We present a common unifying macroscopic framework for precursors in relativistic shock waves. These precursors transfer energy and momentum from the hot downstream to the cold upstream, modifying the shock structure. It was shown that in a steady state, there is a maximal fraction of the downstream energy flux that the precursor can carry. We show here that at this critical value, the shock disappears, and the flow passes through a sonic point. This behaviour resembles the classical Newtonian Rayleigh flow problem. At the critical value, the transition is unstable as perturbations in the upstream accumulate at the sonic point. Thus, if such a point is reached, the shock structure is drastically modified, and the flow becomes turbulent.