[1] INAF, Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
[2] Dipartimento di Fisica, Universita' degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
[3] Laboratory for High Energy Astrophysics, NASA/GSFC, Code 662, Greenbelt, MD 20771, United States
[4] Service d'Astrophysique, DSM/DAPNIA/SAp, CE Saclay, 91191 Gif-sur-Yvette Cedex, France
[5] Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom
[6] Instituto de Astrofisica de Canarias, via Lactea, 38205, La Laguna Tenerife, Spain
[7] Max-Planck-Institüt für Extraterrestrische Physik, Giessenbachstraße, 85741 Garching, Germany
[8] APC, UMR 7164, Université Denis Diderot, 2 place Jussieu, 75005 Paris, France
[9] LUTH, Observatoire de Paris, 92195 Meudon Cedex, France