The implications of TeV detected GRB afterglows for acceleration at relativistic shocks

TL;DR

This paper investigates the maximum electron energies in GRB afterglows, showing that current observations challenge the standard SSC model and suggest the need for larger turbulence scales or alternative mechanisms.

Contribution

It provides new constraints on particle acceleration limits in GRB afterglows based on recent TeV observations and challenges existing models.

Findings

Rapid turbulence damping is ruled out by observations.

Ambient magnetic fields must be weaker or comparable to radiation loss limits.

Klein-Nishina effects hinder the production of observed hard VHE gamma-ray spectra.

Abstract

Motivated by the detection of very high energy gamma-rays deep in the afterglow emission of a gamma-ray burst, we revisit predictions of the maximum energy to which electrons can be accelerated at a relativistic blast wave. Acceleration at the weakly-magnetized forward shock of a blast-wave can be limited either by the rapid damping of turbulence generated behind the shock, by the effect of a large-scale ambient magnetic field, or by radiation losses. Within the confines of a standard, single zone, synchrotron-self-Compton (SSC) model, we show that observations of GRB190829A rule out a rapid damping of the downstream turbulence. Furthermore, simultaneous fits to the X-ray and TeV gamma-ray emission of this object are not possible unless the limit on acceleration imposed by the ambient magnetic field is comparable or weaker than that imposed by radiation losses. This requires the dominant length scale of the turbulence behind the shock to be larger than that implied by particle-in-cell simulations. However, even then, Klein-Nishina effects prevent production of the hard VHE gamma-ray spectrum suggested by observations. Thus, TeV observations of GRB afterglows, though still very sparse, are already in tension with the SSC emission scenario.