Physical parameters of GRB 970508 from its afterglow synchrotron emission

For the case of an adiabatically expanding blast wave, which is applicable to observed gamma-ray burst (GRB) afterglows at late times, we give express ions to infer the physical properties of the afterglow from the measured sp ectral 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, epsilon = 4 10(52) erg/4 pi sr. We also compute the energy density i n electrons and magnetic field. We find that they are 12% and 9%, respectiv ely, of the nucleon energy density.