2013MNRAS.429.3238C

This paper examines the possibility that hadronic processes produce the ≳ 100MeV photons in the prompt phase of gamma-ray bursts (GRBs) observed by the Fermi-Large Area Telescope (LAT). We calculate analytically the radiation from protons and from secondary electron-positron pairs produced by high-energy protons interacting with γ-rays inside of the GRB jet. We consider both photopion and Bethe-Heitler pair production processes to create secondary electrons and positrons that then radiate via inverse Compton and synchrotron processes. We also consider synchrotron radiation from the protons themselves. We calculate the necessary energy in protons to produce typical Fermi-LAT fluxes of a few µJy at 100MeV. For both of the photopion and Bethe-Heitler processes, we find that the required energy in protons is larger than the observed γ-ray energy by a factor of a thousand or more. For proton synchrotron, the protons have a minimum Lorentz factor ∼ 2 x 10⁶. This is much larger than expected if the protons are accelerated by relativistic collisionless shocks in GRBs. We also provide estimates of neutrino fluxes expected from photohadronic processes. Although the flux from a single burst is below IceCube detection limits, it may be possible to rule out photohadronic models by adding up the contribution of several bursts. Therefore, photohadronic processes seem an unlikely candidate for producing the Fermi-LAT radiation during the prompt phase of GRBs.