Dissecting the Energy Budget of a Gamma-Ray Burst Fireball

TL;DR

This paper introduces a method to analyze the energy distribution in gamma-ray burst fireballs by combining prompt emission and early afterglow data, enabling measurement of key physical parameters and efficiency.

Contribution

It presents a novel approach to dissect the GRB fireball energy budget and measure parameters like enthalpy, Lorentz factors, and efficiency directly from observational data.

Findings

Parameters can be derived from data with thermal component and early afterglow features.

Results are weakly dependent on interstellar medium density when composition is specified.

Method enables detailed understanding of GRB jet composition and energy distribution.

Abstract

The jet composition and radiative efficiency of GRBs are poorly constrained from the data. If the jet composition is matter-dominated (i.e. a fireball), the GRB prompt emission spectra would include a dominant thermal component originating from the fireball photosphere, and a non-thermal component presumably originating from internal shocks whose radii are greater than the photosphere radius. We propose a method to directly dissect the GRB fireball energy budget into three components and measure their values by combining the prompt emission and early afterglow data. The measured parameters include the initial dimensionless specific enthalpy density ($\eta$), bulk Lorentz factors at the photosphere radius ($\Gamma_{\rm ph}$) and before fireball deceleration ($\Gamma_0$), the amount of mass loading ($M$), as well as the GRB radiative efficiency ($\eta_\gamma$). All the parameters can be derived from the data for a GRB with a dominant thermal spectral component, a deceleration bump feature in the early afterglow lightcurve, and a measured redshift. The results only weakly depend on the density $n$ of the interstellar medium when the composition ${\cal Y}$ parameter (typically unity) is specified.