Synchrotron radiation from ultra-high energy protons and the Fermi observations of GRB 080916C

TL;DR

This paper proposes that synchrotron radiation from ultra-high energy protons in gamma-ray bursts explains observed delays in high-energy emission and links GRBs to ultra-high energy cosmic rays, with implications for jet properties.

Contribution

It introduces a proton synchrotron model for GRB high-energy emission delays and UHE cosmic ray origins, considering jet dynamics and magnetic fields.

Findings

Proton synchrotron radiation can explain the GeV delay in GRB 080916C.

The model links GRB jets to UHE cosmic rays within 100 Mpc.

Energy requirements depend strongly on jet Lorentz factor, constraining jet properties.

Abstract

Fermi gamma-ray telescope data of GRB 080916C with ~1e55 erg in apparent isotropic gamma-ray energy, show a several second delay between the rise of 100 MeV - GeV radiation compared with keV - MeV radiation. Here we show that synchrotron radiation from cosmic ray protons accelerated in GRBs, delayed by the proton synchrotron cooling timescale in a jet of magnetically-dominated shocked plasma moving at highly relativistic speeds with bulk Lorentz factor Gamma ~ 500, could explain this result. A second generation electron synchrotron component from attenuated proton synchrotron radiation makes enhanced soft X-ray to MeV gamma-ray emission. Long GRBs with narrow, energetic jets accelerating particles to ultra-high energies could explain the Auger observations of UHE cosmic rays from sources within 100 Mpc for nano-Gauss intergalactic magnetic fields. The total energy requirements in a proton synchrotron model are proportional to Gamma^(16/3). This model for GRB 080916C is only plausible if Gamma ~< 500 and the jet opening angle is ~ 1 degree.