Electron-Positron Pair Cascades and Precursors in Gamma-Ray Bursts
  Gamma-ray burst sources with a high luminosity can produce electron-positron pair cascades in their environment as a result of back-scattering of a seed fraction of their original hard spectrum. The resulting spectral modifications offer the possibility of diagnosing not only the compactness parameter of the gamma-ray emitting region but also the baryonic density of the environment external to the burst. 
P. Meszaros (1,2,3), E. Ramirez-Ruiz (1) & M.J. Rees (1) ((1) IoA,   Cambridge University; (2) Pennsylvania State University...