Letter to the Editor

Gamma ray emission from magnetized relativistic GRB outflows

ISDC Data Center for Astrophysics, Chemin d'Écogia 16, 1290 Versoix, Switzerland and Geneva Observatory, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland e-mail: Volodymyr.Savchenko@unige.ch

Received: 15 June 2010
Accepted: 19 July 2010

Abstract

We argue that small pitch angle synchrotron emission provides an important dissipation mechanism that has to be taken into account in models of the formation of relativistic magnetized γ-ray burst (GRB) outflows from newborn black holes and/or magnetars. We show that if the GRB outflow is proton loaded, the spectral energy distribution of this emission is expected to sharply peak in the 0.1-1 MeV energy band. If the small pitch-angle synchrotron emission efficiently cools relativistic particles of the outflow, the emission spectrum below the peak energy is a power law with spectral index α −1, close to the typical spectral index of time-resolved GRB spectra. Otherwise, the low energy spectral index can be as hard as α 0, as observed at the beginning of the GRB pulses. We 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 may be responsible for the GeV component.