Direct GRB fireball Lorentz factor measurements through REM early afterglow observations

Issue Title: Proceedings of the Fifth Stromlo Symposium: Disks, Winds and Jets - From Planets to Quasars The huge energies involved in gamma-ray bursts (GRBs) coupled with the short emission time scales unavoidably imply that the emitting source is moving relativistically, with a speed close to that of light. Here we present the REM telescope observations of the early-time near-infrared light curves of the GRB060418 and GRB060607A afterglows. The detection of the afterglow peak provides for the first time a direct measurement of the initial Lorentz factor Γ^sub 0^ of the radiating material. We find that the emitting region was indeed highly relativistic in the first seconds after the explosions, with Γ^sub 0^400. Comparison with the Lorentz factor as determined at later epochs provides direct evidence that the emitting shell is decelerating and confirms that the afterglow emission is powered by the dissipation of bulk kinetic energy. The deceleration radius was inferred to be R ^sub dec^[asymptotically =]10^sup 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. [PUBLICATION ABSTRACT]