On the magnetization of gamma-ray burst blast waves

TL;DR

This paper models the magnetic field decay in gamma-ray burst blast waves, explaining extended high-energy emission through synchrotron radiation with microturbulence decay, supported by multi-wavelength observations.

Contribution

It introduces a model of microturbulence decay in GRB shocks to explain extended emission, providing a quantitative decay exponent consistent across multiple GRBs.

Findings

Microturbulence decays as t^{-0.4} to t^{-0.5} in GRB shocks.

Synchrotron emission from shock-accelerated electrons explains extended >100 MeV emission.

Model fits multi-wavelength data for four GRBs.

Abstract

The origin of magnetic fields that permeate the blast waves of gamma-ray bursts (GRBs) is a long-standing problem. The present paper argues that in four GRBs revealing extended emission at >100 MeV, with follow-up in the radio, optical and X-ray domains at later times, this magnetization can be described as the partial decay of the micro-turbulence that is generated in the shock precursor. Assuming that the bulk of the extended emission >100 MeV can be interpreted as synchrotron emission of shock accelerated electrons, we model the multi-wavelength light curves of GRB 090902B, GRB 090323, GRB 090328 and GRB 110731A, using a simplified then a full synchrotron calculation with power-law-decaying microturbulence \epsilon_B \propto t^{\alpha_t} (t denotes the time since injection through the shock, in the comoving blast frame). We find that these models point to a consistent value of the decay exponent -0.5 < \alpha_t < -0.4.