Physical Parameters of GRB 970508 and GRB 971214 from Their Afterglow Synchrotron Emission
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Abstract
We have calculated synchrotron spectra of relativistic blast waves and nd predicted characteristic frequencies that are more than an order of magnitude di erent from previous calculations. 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, E \ 3 ] 1052 ergs/4n sr. We also compute the energy density in electrons and magnetic eld. We nd that they are 12% and 9%, respectively, of the nucleon energy density and thus conrm for the rst time that both are close to but below equipartition. For GRB 971214, we discuss the break found in its spectrum by Ramaprakash et al. It can be interpreted either as the peak frequency or as the cooling frequency ; both interpretations have some problems, but on balance the break is more likely to be the cooling frequency. Even when we assume this, our ignorance of the self-absorption frequency and presence or absence of beaming make it impossible to constrain the physical parameters of GRB 971214 very well.