Some Implications of inverse-Compton Scattering of Hot Cocoon Radiation   by relativistic jets in Gamma-Ray Bursts
  Long Gamma-Ray Bursts (GRB) relativistic jets are surrounded by hot cocoons which confine jets during their punch out from the progenitor star. These cocoons are copious sources of X-ray photons that can be and are inverse-Compton (IC) scattered to MeV--GeV energies by electrons in the relativistic jet. We provide detailed estimates for IC flux resulting from various interactions between X-ray photons and the relativistic jet, and describe what we can learn about GRB jets and progenitor stars from the detection (or an upper limit) of these IC scattered photons. 
Pawan Kumar, George F. Smoot
astro-ph.HE