Maximum Energy of Particles Accelerated in GRB Afterglow Shocks

TL;DR

This paper models gamma-ray burst afterglow spectra to investigate the maximum energy particles can attain in relativistic shocks, highlighting how future observations could distinguish acceleration mechanisms.

Contribution

It introduces a spectral modeling framework incorporating PIC-motivated acceleration physics to analyze GRB afterglows and assess their maximum particle energies.

Findings

Low-density short GRBs show a GeV synchrotron cutoff within hours.

Current Fermi-LAT data cannot distinguish acceleration models due to limited photon statistics.

Future MeV-TeV observations could differentiate between acceleration scenarios.

Abstract

Particle acceleration in relativistic collisionless shocks remains an open problem in high-energy astrophysics. Particle-in-cell (PIC) simulations predict that electron acceleration in weakly magnetized shocks proceeds via small-angle scattering, leading to a maximum electron energy significantly below the Bohm limit. This upper bound on electron energy manifests observationally as a characteristic synchrotron cutoff, providing a direct probe of the underlying acceleration physics. Gamma-ray burst (GRB) afterglows offer an exceptional laboratory for testing these predictions. Here, we model the spectral evolution of GRB afterglows during the relativistic deceleration phase, incorporating PIC-motivated acceleration prescriptions and self-consistently computing synchrotron and synchrotron self-Compton emission. We find that low-energy bursts in low-density environments, typical of short GRBs, exhibit a pronounced synchrotron cutoff in the GeV band within minutes to hours after the trigger. Applying our framework to GRB 190114C and GRB 130427A, we find that current observations are insufficient to discriminate between PIC-motivated acceleration and the Bohm limit, primarily due to poor photon statistics in the Fermi-LAT band. Nevertheless, future MeV-TeV afterglow observations can break model degeneracies and place substantially tighter constraints on the mechanisms responsible for particle acceleration in relativistic shocks. To this end, we simulate a fiducial nearby short GRB as a promising probe of the cutoff location, for which the two acceleration scenarios are cleanly distinguishable and the detection of such an event in the near future remains feasible.