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

Abstract

We present results of model fits to afterglow data sets of GRB970508, GRB980703 and GRB070125, characterized by long and broad-band coverage. Themodel 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-lawdensity environment.We showthat the spherical model can accurately describe the data only in the case of GRB970508, for which we find a circumburst medium density n α r-2. 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-fitting 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. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.