Letter to the Editor

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

¹ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy e-mail: emilio.molinari@brera.inaf.it
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

Received: 1 March 2007
Accepted: 25 April 2007

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 . 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.