The upstream magnetic field of collisionless GRB shocks: constraint by Fermi-LAT observations

TL;DR

Fermi-LAT observations of long-lived gamma-ray burst emissions provide new constraints on the magnetic field strength before shocks, indicating significant amplification likely driven by high-energy particle streaming.

Contribution

This study derives tighter constraints on the preshock magnetic field in GRBs using high-energy afterglow data, suggesting magnetic amplification by shock-accelerated particles.

Findings

Preshock magnetic field constrained to 1(n/1cc)^{9/8} mG < B < 10^2(n/1cc)^{3/8} mG.

Magnetic field amplification likely driven by streaming of high-energy shock-accelerated particles.

Constraints are more stringent than previous X-ray afterglow-based limits.

Abstract

Long-lived >100 MeV emission has been a common feature of most Fermi-LAT detected gamma-ray bursts (GRBs), e.g., detected up to ~10^3s in long GRBs 080916C and 090902B and ~10^2s in short GRB 090510. This emission is consistent with being produced by synchrotron emission of electrons accelerated to high energy by the relativistic collisionless shock propagating into the weakly magnetized medium. Here we show that this high-energy afterglow emission constrains the preshock magnetic field to satisfy 1(n/1cc)^{9/8} mG<B<10^2(n/1cc)^{3/8}mG, where n is the preshock density, more stringent than the previous constraint by X-ray afterglow observations on day scale. This suggests that the preshock magnetic field is strongly amplified, most likely by the streaming of high energy shock accelerated particles.