Observations of particle acceleration in the blast waves of Gamma-ray Bursts

TL;DR

This study analyzes X-ray observations of GRB afterglows to determine the distribution of the electron energy index p, finding it is best described by a Gaussian centered at 2.4 with a moderate width, challenging the idea of a universal p value.

Contribution

The paper provides the first observational constraints on the distribution of the electron energy index p in GRB afterglows, showing it is Gaussian rather than a single value.

Findings

The distribution of p is inconsistent with a single discrete value.

The p distribution is well described by a Gaussian centered at 2.4.

The width of the p distribution is approximately 0.6.

Abstract

The electron energy distribution index, p, is a fundamental parameter of the process by which electrons at shock fronts are accelerated to relativistic speeds and by which they radiate, via synchrotron emission. This acceleration process is applicable to a myriad of astronomical jet sources such as AGN, X-ray binaries and gamma-ray bursts (GRBs), as well as to particle acceleration in the solar wind and supernovae, and the acceleration of cosmic rays. The accurate measurement of the distribution of p is of fundamental importance to differentiate between the possible theories of electron acceleration at any relativistic shock front; there is division as to whether p has a universal value or whether it has a distribution, and if so, what that distribution is. Here one such source of synchrotron emission is examined: the blast waves of GRB afterglows observed by the Swift satellite. Within the framework of the GRB blast wave model, the constraints placed on the distribution of p by the observed X-ray spectral and temporal indices are examined and the distribution parametrized. The results show that the observed distribution of spectral indices is inconsistent with an underlying distribution of p composed of a single discrete value but consistent with a Gaussian distribution centred at p = 2.4 and having a width of 0.6. This finding disagrees with theoretical work that argues for a single, universal value of p, but also demonstrates that the width of the distribution is not as wide as has been suggested by some authors.