[1] Univ Bologna, Dipartimento Fis & Astron, Via Gobetti 92-3, I-40121 Bologna, Italy
[2] Osservatorio Astrofis & Sci Spazio, INAF, Via Pietro Gobetti 93-3, I-40121 Bologna, Italy
[3] Ist Nazl Fis Nucl, Sez Bologna, Viale Berti Pichat 6-2, I-40127 Bologna, Italy
[4] Univ Sternwarte, Ludwig Maximilians Univ Munchen, Fak Phys, Scheinerstr 1, D-81679 Munich, Germany
[5] Max Planck Inst Astrophys, Karl Schwarzschild Str 1, D-85741 Garching, Germany
[6] Univ Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany
[7] INAF, Ist Radio Astron, Via Gobetti 101, I-40121 Bologna, Italy
[8] Osserv Astron Trieste, INAF, Via GB Tiepolo 11, I-34143 Trieste, Italy
[9] Inst Fundamental Phys Universe, IFPU, Via Beirut 2, I-34014 Trieste, Italy