Exact versus approximate equitemporal surfaces in Gamma-Ray Burst   afterglows
  By integrating the relativistic hydrodynamic equations introduced by Taub we have determined the exact EQuiTemporal Surfaces (EQTSs) for the Gamma-Ray Burst (GRB) afterglows. These surfaces are compared and contrasted to the ones obtained, using approximate methods, by Panaitescu & Meszaros (1998), Sari (1998), Granot et al. (1999). 
Carlo Luciano Bianco and Remo Ruffini