A semi-analytic afterglow with thermal electrons and synchrotron self-Compton emission

TL;DR

This paper models gamma-ray burst afterglows considering thermal electrons, revealing unique spectral and temporal features, especially in TeV and X-ray emissions, differing significantly from traditional power-law electron models.

Contribution

It introduces a novel semi-analytic model incorporating thermal electron distributions based on first-principles simulations, affecting broadband afterglow predictions.

Findings

Early-time TeV emission is significantly enhanced.

Thermal electrons cause distinctive time-dependent spectral features.

X-ray closure relations are qualitatively different and match observations.

Abstract

We extend previous work on gamma-ray burst (GRB) afterglows involving hot thermal electrons at the base of a shock-accelerated tail. Using a physically-motivated electron distribution based on first-principles simulations, we compute broadband emission from radio to TeV gamma-rays. For the first time, we present the effects of a thermal distribution of electrons on synchrotron self-Compton (SSC) emission. The presence of thermal electrons causes temporal and spectral structure across the entire observable afterglow, which is substantively different from models that assume a pure power-law distribution for the electrons. We show that early-time TeV emission is enhanced by more than an order of magnitude for our fiducial parameters, with a time-varying spectral index that does not occur for a pure power law of electrons. We further show that the X-ray "closure relations" take a very different, also time-dependent, form when thermal electrons are present; the shape traced out by the X-ray afterglows is a qualitative match to observations of the traditional decay phase.