An unexpectedly rapid decline in the X-ray afterglow emission of long   gamma-ray bursts
  Long gamma-ray bursts (GRBs) are commonly accepted to originate in the explosion of particularly massive stars, which gives rise to a highly relativistic jet. Internal inhomogeneities in the expanding flow give rise to internal shock waves that are believed to produce the gamma-rays we see. As the jet travels further outward into the surrounding circumstellar medium `external' shocks give rise to the afterglow emission seen in the X-ray, optical and radio bands. Here we report on the early phases of the X-ray emission of five GRBs. Their X-ray light curves are characterised by a rapid fall-off for the first few hundred seconds, followed by a less rapid decline lasting several hours. This steep decline, together with detailed spectral properties of two particular bursts, shows that violent shock interactions take place in the early jet outflows. 
G. Tagliaferri, M. Goad, G. Chincarini, A. Moretti, S. Campana, D.N.   Burrows, M. Perri, S.D. Barthelmy, N. Gehrels, H. Kri...