2012MNRAS.421L..39B

Photons of energy larger than 100 MeV from long GRBs arrive a few seconds after <10 MeV photons do. We show that this delay is a natural consequence of a magnetic-dominated relativistic jet. The much slower acceleration of a magnetic jet with radius (compared with a hot baryonic outflow) results in high-energy γ-ray photons to be converted to electron-positron pairs out to a larger radius, whereas lower energy γ-rays of energy less than ∼ 10 MeV can escape when the jet crosses the Thomson photosphere. The resulting delay for the arrival of high-energy photons is found to be similar to the value observed by the Fermi satellite for a number of GRBs. A prediction of this model is that the delay should increase with photon energy (E) as ∼ E^0.17 for E > 100 MeV. The delay depends almost linearly on burst redshift, and on the distance from the central compact object where the jet is launched (R₀). Therefore, the delay in arrival of >10² MeV photons can be used to estimate burst redshift if the magnetic jet model for γ-ray generation is correct and R₀ is roughly the same for long GRBs.