Afterglow emission in Gamma-Ray Bursts: I. Pair-enriched ambient medium and radiative blast waves

TL;DR

This paper develops a generalized hydrodynamical model for GRB afterglows, incorporating effects of radiative losses and pair-enrichment, to better understand high-energy emission and light curve decay.

Contribution

It introduces a comprehensive model that accounts for radiative and adiabatic processes, including pair-enrichment, for spherical relativistic blast waves in GRBs.

Findings

Model describes adiabatic, radiative, and semi-radiative blast waves.

Enhanced leptonic energy fraction due to pairs increases radiative losses.

Potential explanation for fast decay in high-energy GRB afterglow light curves.

Abstract

Forward shocks caused by the interaction between a relativistic blast wave and the circum-burst medium are thought to be responsible for the afterglow emission in Gamma-Ray Bursts (GRBs). We consider the hydrodynamics of a spherical relativistic blast wave expanding into the surrounding medium and we generalize the standard theory in order to account for several effects that are generally ignored. In particular, we consider the role of adiabatic and radiative losses on the hydrodynamical evolution of the shock, under the assumption that the cooling losses are fast. Our model can describe adiabatic, fully radiative and semi-radiative blast waves, and can describe the effects of a time-varying radiative efficiency. The equations we present are valid for arbitrary density profiles, and also for a circum-burst medium enriched with electron-positron pairs. The presence of pairs enhances the fraction of shock energy gained by the leptons, thus increasing the importance of radiative losses. Our model allows to study whether the high-energy (>0.1 GeV) emission in GRBs may originate from afterglow radiation. In particular, it is suitable to test whether the fast decay of the high-energy light curve observed in several Fermi LAT GRBs can be ascribed to an initial radiative phase, followed by the standard adiabatic evolution.