A Simple Test of the External Shock Model for the Prompt Emission in   Gamma-Ray Bursts
  It is demonstrated here that if the prompt GRB emission is produced by the simplest version of the external shock model, a specific relation should prevail between the observed duration, isotropic equivalent energy, and photon peak energy. In essence, this relation arises because both the burst duration and the typical energy of the emitted synchrotron photons depend on the same combination of the, usually poorly constrained, external density at the deceleration radius, $n_{dec}$, and initial bulk Lorentz factor, $\Gamma_0$. This has the fortunate consequence of making the relation independent of both $\Gamma_0$ and $n_{dec}$. Unless the efficiency of electron acceleration is very low, synchrotron gamma-rays from the external shock would fail to meet the current observational constraints for the vast majority of GRBs, including those with a smooth, single peak temporal profile. This argues either against an external shock origin for the prompt emission in GRBs or for changes in our understanding of the microphysical and radiation processes occurring within the shocked region. 
Enrico Ramirez-Ruiz, Jonathan Granot