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

Abstract : 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.
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Submitted on : Monday, January 15, 2007 - 12:20:32 PM
Last modification on : Saturday, September 19, 2020 - 4:50:32 AM

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E. Molinari, S.D. Vergani, D. Malesani, S. Covino, P. d'Avanzo, et al.. REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination. Astronomy and Astrophysics - A&A, EDP Sciences, 2007, 469, pp.L13-L16. ⟨10.1051/0004-6361:20077388⟩. ⟨in2p3-00124483⟩

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