Signatures of a Maxwellian Component in Shock-Accelerated Electrons in GRBs

TL;DR

This paper investigates how a Maxwellian electron component in relativistic shocks influences gamma-ray burst emissions, explaining observed steep decay phases and spectral evolution in afterglows.

Contribution

It introduces the observational implications of a mixed thermal-nonthermal electron distribution in GRB shocks, linking simulations to observable signatures.

Findings

Steep decay in X-ray afterglows at ~100 sec post-burst

Hard-soft-hard spectral evolution in early afterglows

Predicted bump in synchrotron peak due to Maxwellian component

Abstract

Recent particle-in-cell simulations suggest that a large fraction of the energy dissipated in a relativistic shock is deposited into a Maxwellian distribution of electrons that is connected to the high-energy power-law tail. Here, we explore the observational implications of such a mixed thermal-nonthermal particle distribution for the afterglow and prompt emission of gamma-ray bursts. When the Maxwellian component dominates the energy budget, the afterglow lightcurves show a very steep decline phase followed by a more shallow decay when the characteristic synchrotron frequency crosses the observed band. The steep decay appears in the X-rays at ~100 sec after the burst and is accompanied by a characteristic hard-soft-hard spectral evolution that has been observed in a large number of early afterglows. If internal shocks produce a similar mixed electron distribution, a bump is expected at the synchrotron peak of the nu*f_nu spectrum.