[1] Univ Torino, Dipartimento Fis, Via Pietro Giuria 1, I-10125 Turin, Italy
[2] Fdn Res & Technol Hellas, Inst Astrophys, Voutes 7110, Heraklion, Greece
[3] INAF Osservatorio Astrofisico Torino, Via Osservatorio 20, I-10025 Pino Torinese, Italy
[4] Ist Nazl Fis Nucleare, Sez Torino, I-10125 Turin, Italy
[5] Consorzio Interuniv Fis Spaziale, Via Pietro Giuria 1, I-10125 Turin, Italy
[6] INAF Ist Radioastron, Via Piero Gobetti 101, I-40129 Bologna, Italy
[7] Harvard & Smithsonian, Ctr Astrophys, 60 Garden St, Cambridge, MA 02138 USA
[8] Univ Napoli Federico II, Dipartimento Fis Ettore Pancini, Via Cintia, I-80126 Naples, Italy
[9] Univ Geneva, Chemin Ec 16, CH-1290 Versoix, Switzerland
[10] INAF OAS, Via Piero Gobetti 101, I-40129 Bologna, Italy
[11] INFN, Sez Bologna, Viale Berti Pichat 6-2, I-40127 Bologna, Italy
[12] INAF Osservatorio Astron Palermo G S Vaiana, Piazza Parlamento 1, I-90134 Palermo, Italy