On the Electron Energy Distribution Index of Swift Gamma-ray Burst Afterglows

Abstract

The electron energy distribution index, p, is a fundamental parameter of the synchrotron emission from a rangeof astronomical sources. Here we examine one such source of synchrotron emission, gamma-ray burst (GRB)afterglows observed by the Swift satellite. Within the framework of the blast wave model, we examine theconstraints placed on the distribution of p by the observed X-ray spectral indices and parameterize the distribution.We find that the observed distribution of spectral indices are inconsistent with an underlying distribution ofp composed of a single discrete value but consistent with a Gaussian distribution centered at p = 2.36 andhaving a width of 0.59. Furthermore, accepting that the underlying distribution is a Gaussian, we find that themajority ( 94%) of GRB afterglows in our sample have cooling break frequencies less than the X-ray frequency.