Letter to the Editor
REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination*
INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy e-mail: firstname.lastname@example.org
2 Dunsink Observatory - DIAS, Dunsink lane, Dublin 15, Ireland
3 School of Physical Sciences and NCPST, Dublin City University – Dublin 9, Ireland
4 International School for Advanced Studies (SISSA/ISAS), via Beirut 2-4, 34014 Trieste, Italy
5 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 2100 København, Denmark
6 Dipartimento di Fisica e Matematica, Università dell'Insubria, via Valleggio 11, 22100 Como, Italy
7 Università degli Studi di Milano Bicocca, piazza delle Scienze 3, 20126 Milano, Italy
8 INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone (Roma), Italy
9 Dipartimento di Fisica e Osservatorio Astronomico, Università di Perugia, via A. Pascoli, 06123 Perugia, Italy
10 INAF-IASF di Bologna, via P. Gobetti 101, 40129 Bologna, Italy
11 APC, Laboratoire Astroparticule et Cosmologie, UMR 7164, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France
12 CEA Saclay, DSM/DAPNIA/Service d'Astrophysique, 91191 Gif-sur-Yvette, France
13 CNRS, Institut Nel, 25 rue des Martyrs, 38042 Grenoble, France
14 INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy
15 European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
Accepted: 25 April 2007
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 . We aim to provide a direct measure of .
Methods.The early-time afterglow light curve carries information about , 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.
Results.For both events, the afterglow peak could be clearly singled out, allowing a firm determination of the fireball Lorentz of 400, fully confirming the highly relativistic nature of GRB fireballs. The deceleration radius was inferred to be 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.
Key words: gamma rays: bursts / relativity
© ESO, 2007