High Energy Photons From Gamma Ray Bursts

Emission of high energy (HE) photons above 100 MeV that is delayed and lasts much longer than the prompt MeV emission has been detected from several long duration gamma ray bursts (LGRBs) and short hard bursts (SHBs) by the Compton, Fermi and AGILE gamma ray observatories. In this paper we show that the main observed properties of this HE emission are those predicted by the cannonball (CB) model of GRBs: In the CB model all the observed radiations in a GRB are produced by the interaction of a highly relativistic jet of plasmoids (CBs) with the environment. The prompt X-ray and MeV $\gamma$-ray pulses are produced by inverse Compton scattering (ICS) of glory photons -photons scattered/emitted into a cavity created by the wind/ejecta blown from the progenitor star or a companion star long before the GRB- by the thermal electrons in the CBs. A simultaneous optical and high energy emission begins shortly after each MeV pulse when the CB collides with the wind/ejecta, and continues during the deceleration of the CB in the interstellar medium. The optical emission is dominated by synchrotron radiation (SR) from the swept-in and knocked-on electrons which are Fermi accelerated to high energies by the turbulent magnetic fields in the CBs, while ICS of these SR photons dominates the emission of HE photons. The lightcurves of the optical and HE emissions have approximately the same temporal behaviour but have different power-law spectra. The emission of very high energy (VHE) photons above 100 TeV is dominated by the decay of $\pi^0$'s produced in hadronic collisions of Fermi accelerated protons in the CBs. The CB model explains well all the observed radiations, including the high energy radiation from both LGRBs and SHBs as demonstrated here for GRB 090902B and SHB 090510.