2011MNRAS.411..422L

We consider dynamical scales in magnetized gamma-ray burst (GRB) outflows, using the solutions to the Riemann problem of expanding arbitrarily magnetized outflows. For high ejecta magnetization, the behaviour of the forward shock closely resembles the so-called thick-shell regime of the hydrodynamical expansion. The exception is at small radii, where the motion of the forward shock is determined by the dynamics of subsonic relativistic outflows. The behaviours of the reverse shock is different in fluid and magnetized cases: in the latter case, even for medium magnetization, σ ∼ 1, the reverse shock forms at fairly large distances, and may never form in a wind-type external density profile.