Challenging gamma-ray burst models through the broadband dataset of GRB 060908

¹ INAF/Osservatorio Astronomico di Brera, via Emilio Bianchi 46, 23807 Merate (LC), Italy e-mail: stefano.covino@brera.inaf.it
² Harvard-Smithsonian Center for Astrophysics, MA 02138, USA
³ INAF/Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone (Roma), Italy
⁴ INAF/Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy
⁵ APC, Laboratoire Astroparticule et Cosmologie, UMR 7164, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France
⁶ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique, 91191 Gif-sur-Yvette, France
⁷ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁸ Department of Astronomy & Astrophysics, 525 Davey Lab., Pennsylvania State University, University Park, PA 16802, USA
⁹ Department of Astronomy, 601 Campbell Hall, University of California, Berkeley, CA 94720–3411, USA
¹⁰ Instituto de Fisica de Cantabria (CSIC-UC), Av. los Castros s/n, 39005 Santander, Spain
¹¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹² Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 2100 Copenhagen, Denmark
¹³ Institut de Radio Astronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin-d'Hères, France
¹⁴ Instituto de Astrofísica de Andalucía, PO Box 3.004, 18080 Granada, Spain
¹⁵ INAF/TNG Fundación Galileo Galilei – Rambla José Ana Fernández Pérez, 7, 38712 Breña Baja, TF – Spain
¹⁶ Observatori Astronomic de la Universitat de València, Paterna-46980, Valencia, Spain
¹⁷ Dipartimento di Fisica e Osservatorio Astronomico, Università di Perugia, via A. Pascoli, 06123 Perugia, Italy
¹⁸ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹⁹ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavk, Iceland
²⁰ INAF/Catania Astrophysical Observatory, via S. Sofia 78, 95123 Catania, Italy
²¹ Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen, Denmark
²² SUPA Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY 9SS, Scotland, UK
²³ Dunsink Observatory – DIAS, Dunsink Lane, Dublin 15, Ireland
²⁴ INAF/Osservatorio Astronomico di Trieste, via G. Tiepolo 11, 34143 Trieste, Italy
²⁵ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy

Received: 15 May 2010
Accepted: 26 July 2010

Abstract

Context. Multiwavelength observations of gamma-ray burst prompt and afterglow emission are a key tool to separate the various possible emission processes and scenarios proposed to interpret the complex gamma-ray burst phenomenology.

Aims. We collected a large dataset on GRB 060908 in order to carry out a comprehensive analysis of the prompt emission as well as the early and late afterglow.

Methods. Data from Swift-BAT, -XRT and -UVOT together with data from a number of different ground-based optical/near-infrared and millimeter telescopes allowed us to follow the afterglow evolution after about a minute from the high-energy event down to the host galaxy limit. We discuss the physical parameters required to model these emissions.

Results. The prompt emission of GRB 060908 was characterised by two main periods of activity, spaced by a few seconds of low intensity, with a tight correlation between activity and spectral hardness. Observations of the afterglow began less than one minute after the high-energy event, when it was already in a decaying phase, and it was characterised by a rather flat optical/near-infrared spectrum which can be interpreted as due to a hard energy-distribution of the emitting electrons. On the other hand, the X-ray spectrum of the afterglow could be fit by a rather soft electron distribution.

Conclusions. GRB 060908 is a good example of a gamma-ray burst with a rich multi-wavelength set of observations. The availability of this dataset, built thanks to the joint efforts of many different teams, allowed us to carry out stringent tests for various interpretative scenarios, showing that a satisfactorily modelling of this event is challenging. In the future, similar efforts will enable us to obtain optical/near-infrared coverage comparable in quality and quantity to the X-ray data for more events, therefore opening new avenues to progress gamma-ray burst research.