Probes of Diffusive Shock Acceleration using Gamma-Ray Burst Prompt Emission

TL;DR

This paper investigates how gamma-ray burst prompt emission spectra can reveal details about relativistic shock acceleration, focusing on spectral features above 1 MeV to understand magnetic field obliquity and turbulence.

Contribution

It introduces spectral diagnostics to constrain electron acceleration properties in GRB internal shocks, emphasizing the role of high-energy spectra and broadband observations.

Findings

Electrons responsible for prompt emission are mainly non-thermal.

Synchrotron emission is favored over self-Compton for GRB spectra.

Spectral features above 1 MeV probe shock magnetic field obliquity.

Abstract

The principal paradigm for gamma-ray bursts (GRBs) suggests that the prompt transient gamma-ray signal arises from multiple shocks internal to the relativistic expansion. This paper explores how GRB prompt emission spectra can constrain electron (or ion) acceleration properties at the relativistic shocks that pertain to GRB models. The array of possible high-energy power-law indices in accelerated populations is highlighted, focusing on how spectra above 1 MeV can probe the field obliquity in GRB internal shocks, and the character of hydromagnetic turbulence in their environs. When encompassing the MeV-band spectral break, fits to BATSE/EGRET burst data indicate that the preponderance of electrons responsible for the prompt emission reside in an intrinsically non-thermal population. This differs markedly from typical populations generated in acceleration simulations; potential resolutions of this conflict such as the action of self-absorption are mentioned. Spectral modeling also suggests that the synchrotron mechanism is favored over synchrotron self-Compton scenarios due to the latter's typically broad curvature near the peak. Such diagnostics will be enhanced by the broadband spectral coverage of bursts by the Fermi Gamma-Ray Space Telescope; the GBM will provide key information on the lower energy portions of the non-thermal particle population, while the LAT will constrain the power-law regime of particle acceleration.