[1] Univ Torino, Dipartimento Fis, Via Pietro Giuria 1, I-10125 Turin, Italy
[2] Ist Nazl Fis Nucl, Sez Torino, I-10125 Turin, Italy
[3] INAF Osserv Astrofis Torino, Via Osservatorio 20, I-10025 Pino Torinese, Italy
[4] Consorzio Interuniv Fis Spaziale, Via Pietro Giuria 1, I-10125 Turin, Italy
[5] Univ La Laguna, Dept Astrofis, Tenerife 38206, Spain
[6] Inst Astrofis Canarias IAC, Tenerife 38200, Spain
[7] Univ Bologna, ADipartimento Fis & Astron, Via Piero Gobetti 93-2, I-40129 Bologna, Italy
[8] INAF, Ist Radioastron, Via Gobetti 101, I-40129 Bologna, Italy
[9] Pontificia Univ Catolica Chile, Fac Fis, Inst Astrofis, Casilla 306, Santiago 22, Chile
[10] Pontificia Univ Catolica Chile, Fac Fis, Centro Astroingn, Casilla 306, Santiago 22, Chile
[11] Harvard & Smithsonian, Ctr Astrophys, 60 Garden St, Cambridge, MA 02138 USA
[12] Univ Manitoba, Dept Phys & Astron, Winnipeg, MB R3T 2N2, Canada
[13] Rochester Inst Technol, Ctr Imaging Sci, 84 Lomb Mem Dr, Rochester, NY 14623 USA
[14] Rochester Inst Technol, Sch Phys & Astron, 84 Lomb Mem Dr, Rochester, NY 14623 USA