Gamma-ray burst afterglows as probes of environment and blast wave physics. II. The distribution of p and structure of the circumburst medium

We constrain blast wave parameters and the circumburst media of a subsample of 10 BeppoSAX gamma-ray bursts (GRBs). For this sample we derive the values of the injected electron energy distribution index, p, and the density structure index of the circumburst medium, k, from simultaneous spectral fits to their X-ray, optical, and NIR afterglow data. The spectral fits have been done in count space and include the effects of metallicity, and are compared with the previously reported optical and X-ray temporal behavior. Using the blast wave model and some assumptions which include on-axis viewing and standard jet structure, constant blast wave energy, and no evolution of the microphysical parameters, we find a mean value of p for the sample as a whole of 2.04(-0.03)(+0.02). A statistical analysis of the distribution demonstrates that the p-values in this sample are inconsistent with a single universal value for p at the 3 sigma level or greater, which has significant implications for particle acceleration models. This approach provides us with a measured distribution of circumburst density structures rather than considering only the cases of k 0 (homogeneous) and k 2 (windlike). We find five GRBs for which k can be well constrained, and in four of these cases the circumburst medium is clearly windlike. The fifth source has a value of 0 <= k <= 1, consistent with a homogeneous circumburst medium.