Magnetization Degree of Gamma-Ray Burst Fireballs: Numerical Study

TL;DR

This study uses numerical simulations to reveal that gamma-ray burst fireball magnetization is significantly underestimated by standard methods, especially in the reverse shock regime, leading to revised estimates of magnetic field strengths.

Contribution

The paper introduces improved analytic estimates for reverse shock emission and demonstrates that fireball magnetization is stronger than previously thought, especially in the sub-relativistic regime.

Findings

Standard treatment underestimates magnetization by up to two orders of magnitude.

Reverse shock temperature remains constant at about 0.08 when  exceeds several.

Magnetic fields in GRB fireballs are stronger than earlier estimates.

Abstract

The relative strength between forward and reverse shock emission in early gamma-ray burst afterglow reflects that of magnetic energy densities in the two shock regions. We numerically show that with the current standard treatment, the fireball magnetization is underestimated by up to two orders of magnitude. This discrepancy is especially large in the sub-relativistic reverse shock regime (i.e. the thin shell and intermediate regime) where most optical flashes were detected. We provide new analytic estimates of the reverse shock emission based on a better shock approximation, which well describe numerical results in the intermediate regime. We show that the reverse shock temperature at the onset of afterglow is constant, $(\bar{\Gamma}_d-1)\sim 8\times10^{-2}$, when the dimensionless parameter $\xi_{0}$ is more than several. Our approach is applied to case studies of GRB 990123 and 090102, and we find that magnetic fields in the fireballs are even stronger than previously believed.