On particle acceleration rate in GRB afterglows

TL;DR

This study uses gamma-ray burst afterglow observations to constrain the particle acceleration rate in relativistic collisionless shocks, finding it to be very fast, near the Bohm limit, especially in ultra-relativistic conditions.

Contribution

It provides the first observational constraints on particle acceleration rates in GRB afterglows, suggesting acceleration is rapid and close to the Bohm limit during the ultra-relativistic phase.

Findings

Acceleration rate is likely very fast, near the Bohm limit.

X-ray afterglow decay can be explained by inability to accelerate electrons at late times.

Acceleration cannot be more than 100 times slower than the Bohm limit.

Abstract

It is well known that collisionless shocks are major sites of particle acceleration in the Universe, but the details of the acceleration process are still not well understood. The particle acceleration rate, which can shed light on the acceleration process, is rarely measured in astrophysical environments. Here we use observations of gamma-ray burst afterglows, which are weakly magnetized relativistic collisionless shocks in ion-electron plasma, to constrain the rate of particle acceleration in such shocks. We find, based on X-ray and GeV afterglows, an acceleration rate that is most likely very fast, approaching the Bohm limit, when the shock Lorentz factor is in the range of 10-100. In that case X-ray observations may be consistent with no amplification of the magnetic field in the shock upstream region. We examine the X-ray afterglow of GRB 060729, which is observed for 642 days showing a sharp decay in the flux starting about 400 days after the burst, when the shock Lorentz factor is about 5. We find that inability to accelerate X-ray emitting electrons at late time provides a natural explanation for the sharp decay, and that also in that case acceleration must be rather fast, and cannot be more than a 100 times slower than the Bohm limit. We conclude that particle acceleration is most likely fast in GRB afterglows, at least as long as the blast wave is ultra-relativistic.