# Nonlinear particle acceleration and thermal particles in GRB afterglows

## TL;DR

This paper investigates how including thermal particles and nonlinear acceleration effects in GRB afterglow models alters the predicted spectra and light curves, significantly impacting observability with future gamma-ray telescopes.

## Contribution

It introduces a hydrodynamical simulation framework that incorporates thermal particles and nonlinear effects, revealing deviations from the standard power-law electron spectrum in GRB afterglows.

## Key findings

- Electron spectra are not well-fitted by a single power law when thermal particles are included.
- Efficient acceleration reduces the size of the power-law region in the spectrum.
- Thermal particle emission significantly enhances optical and GeV fluxes, improving detection prospects.

## Abstract

The standard model for GRB afterglow emission treats the accelerated electron population as a simple power law, $N(E) \propto E^{-p}$ for $p \gtrsim 2$. However, in standard Fermi shock acceleration a substantial fraction of the swept-up particles do not enter the acceleration process at all. Additionally, if acceleration is efficient then the nonlinear backreaction of accelerated particles on the shock structure modifies the shape of the non-thermal tail of the particle spectra. Both of these modifications to the standard synchrotron afterglow impact the luminosity, spectra, and temporal variation of the afterglow. To examine the effects of including thermal particles and nonlinear particle acceleration on afterglow emission, we follow a hydrodynamical model for an afterglow jet and simulate acceleration at numerous points during the evolution. When thermal particles are included, we find that the electron population is at no time well-fitted by a single power law, though the highest-energy electrons are; if the acceleration is efficient, then the power law region is even smaller. Our model predicts hard-soft-hard spectral evolution at X-ray energies, as well as an uncoupled X-ray and optical light curve. Additionally, we show that including emission from thermal particles has drastic effects (factors of 100 and 30, respectively) on the observed flux at optical and GeV energies. This enhancement of GeV emission makes afterglow detections by future $\gamma$-ray observatories, such as CTA, very likely.