Dynamics of strongly magnetized ejecta in gamma-ray bursts

We consider dynamical scales in magnetized GRB outflows, using the solutions to the Riemann problem of expanding arbitrarily magnetized outflows (Lyutikov 2010). For high ejecta magnetization, the behavior 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 behaviors 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. 1