REM observations of GRB060418 and GRB060607A: the onset of the afterglow and the initial fireball Lorentz factor determination: The onset of the afterglow and the initial fireball Lorentz factor determination

E. Molinari, S. D. Vergani, D. Malesani, S. Covino, P. D'Avanzo, G. Chincarini, F. M. Zerbi, L. A. Antonelli, P. Conconi, V. Testa, G. Tosti, F. Vitali, F. D'Alessio, G. Malaspina, L. Nicastro, E. Palazzi, D. Guetta, S. Campana, P. Goldoni, N. MasettiE. J. A. Meurs, A. Monfardini, L. Norci, E. Pian, S. Piranomonte, D. Rizzuto, M. Stefanon, L. Stella, G. Tagliaferri, P. A. Ward, G. Ihle, L. Gonzalez, A. Pizarro, P. Sinclaire, J. Valenzuela

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Abstract

Context. Gamma-ray burst (GRB) emission is believed to originate in highly relativistic fireballs. Aims. Currently, only lower limits were securely set to the initial fireball Lorentz factor Γ0 We aim to provide a direct measure of Γ0. Methods. The early-time afterglow light curve carries information about Γ0, which determines the time of the afterglow peak. We have obtained early observations of the near-infrared afterglows of GRB 060418 and GRB 060607 A with the REM robotic telescope. Results. For both events, the afterglow peak could be clearly singled out, allowing a firm determination of the fireball Lorentz of Rdec ≈ 400, fully confirming the highly relativistic nature of GRB fireballs. The deceleration radius was inferred to be Rdec ≈ 1017 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.

Original languageEnglish
Pages (from-to)L13-U16
JournalAstronomy and Astrophysics
Volume469
Issue number1
DOIs
StatePublished - Jul 2007
Externally publishedYes

Keywords

  • gamma rays : bursts
  • relativity

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