REM observations of GRB 060418 and GRB 060607A: the onset of the   afterglow and the initial fireball Lorentz factor determination
  Gamma-ray burst (GRB) emission is believed to originate in highly relativistic fireballs. Currently, only lower limits were securely set to the initial fireball Lorentz factor Gamma_0. We aim to provide a direct measure of Gamma_0. The early-time afterglow light curve carries information about Gamma_0, which determines the time of the afterglow peak. We have obtained early observations of the near-infrared afterglows of GRB 060418 and GRB 060607A with the REM robotic telescope. For both events, the afterglow peak could be clearly singled out, allowing a firm determination of the fireball Lorentz of Gamma_0 ~ 400, fully confirming the highly relativistic nature of GRB fireballs. The deceleration radius was inferred to be R_dec ~ 10^17 cm. This is much larger than the internal shocks radius (believed to power the prompt emission), thus providing further evidence for a different origin of the prompt and afterglow stages of the GRB. 
E. Molinari, S.D. Vergani, D. Malesani, S. Covino, P. D'Avanzo, G.   Chincarini, F.M. Zerbi, L.A. Antonelli, P. Conconi, V. ...