Magnetized GRB outflow model: weak reverse shock emission and short energy transfer timescale

TL;DR

This paper explains the lack of bright optical flashes in most Swift GRB afterglows by considering magnetization levels in the reverse shock, showing how different magnetization regimes affect emission and energy transfer.

Contribution

It introduces a magnetized outflow model for GRB afterglows, analyzing how varying magnetization impacts reverse shock emission and energy transfer timescales.

Findings

Weak reverse shock emission can be explained by magnetization with σ ~ 1.

Soft electron spectra occur in mildly magnetized reverse shocks.

High magnetization (σ >> 1) leads to rapid energy transfer incompatible with observed shallow declines.

Abstract

We show that the absence of the bright optical flashes in most {\it Swift} Gamma-ray Burst (GRB) afterglows can be explained, if the reverse shock region is magnetized with a $\sigma \sim 1$, or the emission spectrum of the electrons accelerated in the mildly magnetized ($0.1<\sigma<1$) reverse shock front is very soft, or the reverse shock of a non-magnetized outflow is sub-relativistic, where $\sigma$ is the ratio of the magnetic energy flux to the particle energy flux. We also find that for $\sigma\gg 1$, the energy transfer between the magnetized ejecta and the forward shock may be too quick to account for the shallow decline phase that is well detected in many {\it Swift} GRB X-ray afterglows.