Gamma ray emission from magnetized relativistic GRB outflows

TL;DR

This paper proposes that small pitch angle synchrotron emission is a key dissipation process in magnetized GRB outflows, influencing their spectra and potentially explaining observed gamma-ray features.

Contribution

It introduces the role of small pitch angle synchrotron emission in GRB outflows and its impact on spectral characteristics, a novel aspect in GRB modeling.

Findings

Spectral peak expected in 0.1-1 MeV range for proton-loaded outflows.

Low energy spectral index can be around -1 or 0 depending on cooling efficiency.

Proposes contribution of this emission to the Band component and proton-initiated cascades to GeV emission.

Abstract

We argue that small pitch angle synchrotron emission provides an important dissipation mechanism which has to be taken into account in the models of formation of relativistic magnetized gamma-ray burst (GRB) outflows from the newborn black holes and/or magnetars. We show that if the GRB outflow is proton loaded, spectral energy distribution of this emission is expected to sharply peak in 0.1-1~MeV energy band. If the small pitch angle synchrotron emission efficiently cools relativistic particles of the outflow, its spectrum below the peak energy is a powerlaw with spectral index alpha ~ -1, close to the typical spectral index of the time-resolved GRB spectra. Otherwise, the low energy spectral index can be as hard as alpha ~ 0, as observed at the beginning of the GRB pulses. We make a conjecture that small pitch angle synchrotron emission from proton-loaded magnetized GRB outflow could significantly contribute to the Band component of the prompt emission of GRBs while electromagnetic cascade initiated by the protons could be responsible for the GeV component.