Gamma Ray Bursts from Interaction of Relativistic Flows with Radiation   Fields
  Relativistic flows resulting from sudden explosive events upscatter ambient interstellar photons of local radiation fields. For Lorentz factor $ > 100$ and dense optical - UV radiation fields the emergent signal is a typical gamma ray burst. Presumably the explosions occur in dense globular clusters or in galactic nuclei, at cosmological distances. 
Amotz Shemi