Dynamics of strongly magnetized ejecta in Gamma Ray Bursts

TL;DR

This paper analyzes the dynamics of highly magnetized ejecta in Gamma Ray Bursts, revealing how strong magnetization influences shock behavior and the formation of reverse shocks in different external density profiles.

Contribution

It provides a detailed analysis of magnetized GRB outflows using Riemann problem solutions, highlighting differences from hydrodynamical models and the conditions for reverse shock formation.

Findings

Forward shock behavior resembles thick shell hydrodynamics at high magnetization.

Reverse shock forms at large distances or may not form in wind environments.

Small radii dynamics are governed by subsonic relativistic outflows.

Abstract

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, sigma ~ 1, the reverse shock forms at fairly large distances, and may never form in a wind-type external density profile.