REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination

Context: Gamma-ray burst (GRB) emission is believed to originate in highly relativistic fireballs. <BR />Aims: Currently, only lower limits were securely set to the initial fireball Lorentz factor Γ_0. We aim to provide a direct measure of Γ_0. <BR />Methods: The early-time afterglow light curve carries information about Γ_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. <BR />Results: For both events, the afterglow peak could be clearly singled out, allowing a firm determination of the fireball Lorentz of Γ_0˜ 400, fully confirming the highly relativistic nature of GRB fireballs. The deceleration radius was inferred to be R_dec ≈ 10<SUP>17</SUP> 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. Tables 2 and 3 are only available in electronic form at http://www.aanda.org