Using Gamma-Ray Burst Prompt Emission to Probe Relativistic Shock Acceleration

TL;DR

This paper investigates how gamma-ray burst prompt emission spectra can reveal details about relativistic shock acceleration processes, focusing on spectral indices and turbulence, using data from the Fermi telescope.

Contribution

It links GRB prompt spectra to shock acceleration properties, emphasizing how high-energy observations can probe magnetic field obliquity and turbulence in relativistic shocks.

Findings

Spectral indices vary widely, reflecting diverse shock conditions.

High-energy spectra above 1 MeV can diagnose magnetic field obliquity.

Fermi LAT data constrains non-thermal particle distributions.

Abstract

It is widely accepted that the prompt transient signal in the 10 keV - 10 GeV band from gamma-ray bursts (GRBs) arises from multiple shocks internal to the ultra-relativistic expansion. The detailed understanding of the dissipation and accompanying acceleration at these shocks is a currently topical subject. This paper explores the relationship between GRB prompt emission spectra and the electron (or ion) acceleration properties at the relativistic shocks that pertain to GRB models. The focus is on the array of possible high-energy power-law indices in accelerated populations, highlighting how spectra above 1 MeV can probe the field obliquity in GRB internal shocks, and the character of hydromagnetic turbulence in their environs. It is emphasized that diffusive shock acceleration theory generates no canonical spectrum at relativistic MHD discontinuities. This diversity is commensurate with the significant range of spectral indices discerned in prompt burst emission. Such system diagnostics are now being enhanced by the broadband spectral coverage of bursts by the Fermi Gamma-Ray Space Telescope; while the Gamma-Ray Burst Monitor (GBM) provides key diagnostics on the lower energy portions of the particle population, the focus here is on constraints in the non-thermal, power-law regime of the particle distribution that are provided by the Large Area Telescope (LAT).