Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-12-20
Astron.Astrophys.469:L13-L16,2007
Astronomy and Astrophysics
Astrophysics
6 pages, 3 color figures, 3 tables (including two online); discussion significantly extended, results unchanged; accepted for
Scientific paper
10.1051/0004-6361:20077388
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.
Antonelli Lucio Angelo
Campana Sergio
Chincarini Guido
Conconi Paolo
Covino Stefano
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