On the mediation of a shock front by Fermi-accelerated cosmic rays

Fermi acceleration of high-energy cosmic ray particles by the converging fluid flows on either side of a shock front is investigated. The effect of the preshock cosmic ray pressure in mediating the shock compression and thereby the particle acceleration is studied for a shock in which this pressure can be regarded as a perturbation. The resulting spectral distortions are dictated by the energy dependence of the spatial diffusion coefficient and the Mach number of the shock. Examples are computed for diffusion coefficients independent of and proportional to the particle momentum. The implications of these results for the efficiency of the particle acceleration mechanism are briefly discussed.