Applying an accurate spherical model to gamma-ray burst afterglow observations

TL;DR

This paper introduces a highly accurate spherical model for gamma-ray burst afterglow analysis, combining hydrodynamic simulation precision with flexible formulas to interpret broadband observational data.

Contribution

It presents a novel model that accurately describes the dynamical transition of GRB afterglows from relativistic to Newtonian speeds, incorporating broadband data fitting and physical parameter inference.

Findings

The model accurately fits GRB970508 data and suggests a stellar wind environment.

Evidence for equipartition between electrons and magnetic fields in the afterglow.

Constraints on electron acceleration efficiency for adiabatic blast waves.

Abstract

We present results of model fits to afterglow data sets of GRB970508, GRB980703 and GRB070125, characterized by long and broadband coverage. The model assumes synchrotron radiation (including self-absorption) from a spherical adiabatic blast wave and consists of analytic flux prescriptions based on numerical results. For the first time it combines the accuracy of hydrodynamic simulations through different stages of the outflow dynamics with the flexibility of simple heuristic formulas. The prescriptions are especially geared towards accurate description of the dynamical transition of the outflow from relativistic to Newtonian velocities in an arbitrary power-law density environment. We show that the spherical model can accurately describe the data only in the case of GRB970508, for which we find a circumburst medium density consistent with a stellar wind. We investigate in detail the implied spectra and physical parameters of that burst. For the microphysics we show evidence for equipartition between the fraction of energy density carried by relativistic electrons and magnetic field. We also find that for the blast wave to be adiabatic, the fraction of electrons accelerated at the shock has to be smaller than 1. We present best-fit parameters for the afterglows of all three bursts, including uncertainties in the parameters of GRB970508, and compare the inferred values to those obtained by different authors.