Cosmological GRBs: Internal vs. External Shocks
  Internal (IS) or external (ES) shocks in relativistic expanding shells are currently the best known mechanism for producing GRBs. We calculate the hydrodynamic conditions and the cooling processes in internal shocks, that occur when one layer overtakes another. We compare these conditions with the conditions in external shocks that occur when the shell encounters the ISM. We find that the complicated temporal structure observed in GRBs could be easily explained in IS scenario. Unlike ES, the observed temporal structure simply reflects the temporal behavior of the internal engine that produces the shell. We also find that opacity and in particular the opacity for pair production poses strong constraints on the parameter space and consequently bursts with very narrow peaks are expected to be optically thick and would not contain GeV photons. Finally we find that as in ES synchrotron is the most likely radiation process. 
Re'em Sari and Tsvi Piran (Hebrew University)