# Thermal Electrons in GRB Afterglows

## TL;DR

This paper explores how including thermal electrons in gamma-ray burst afterglow models affects synchrotron radiation, revealing increased opacity and a brighter optical peak that persists over days.

## Contribution

It introduces the role of thermal, non-accelerated electrons in afterglow modeling, which was previously neglected, showing their impact on spectra and light curves.

## Key findings

- Thermal electrons increase synchrotron self-absorption frequency by 10-100 times.
- Additional emission from thermal electrons peaks in optical, brighter than nonthermal emission.
- Thermal electron effects are detectable days after the burst.

## Abstract

To date, nearly all multi-wavelength modeling of long-duration gamma-ray bursts has ignored synchrotron radiation from the significant population of electrons expected to pass the shock without acceleration into a power-law distribution. We investigate the effect of including the contribution of thermal, non-accelerated electrons to synchrotron absorption and emission in the standard afterglow model, and show that these thermal electrons provide an additional source of opacity to synchrotron self-absorption, and yield an additional emission component at higher energies. The extra opacity results in an increase in the synchrotron self-absorption frequency by factors of 10--100 for fiducial parameters. The nature of the additional emission depends on the details of the thermal population, but is generally observed to yield a spectral peak in the optical brighter than radiation from the nonthermal population by similar factors a few seconds after the burst, remaining detectable at millimeter and radio frequencies several days later.