Physical parameters of GRB 970508 from its afterglow synchrotron emission

Abstract

For the case of an adiabatically expanding blast wave, which is applicable to observed gamma-ray burst (GRB) afterglows at late times, we give expressions to infer the physical properties of the afterglow from the measured spectral features. We show that enough data exist for GRB 970508 to compute unambiguously the ambient density, n = 0.03 cm-3, and the blast wave energy per unit solid angle, ε = 4 1052 erg/4π sr. We also compute the energy density in electrons and magnetic field. We find that they are 12% and 9%, respectively, of the nucleon energy density.