Synchrotron Radiation from Ultra-High Energy Protons and the Fermi Observations of GRB 080916C

ABSTRACT

Fermi γ -ray telescope data of GRB 080916C with ~10⁵⁵ erg in apparent isotropic γ -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 Γ ~ 500 , could explain this result. A second generation electron synchrotron component from attenuated proton synchrotron radiation makes enhanced soft X-ray to MeV γ -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 αΓ^16/3 . This model for GRB 080916C is only plausible if Γ≤500 and the jet opening angle is ~1" .

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