1. INAF - Istituto di Astrofisica e Planetologia Spaziali, via del Fosso del Caveliere 100, I-00133 Roma, Italy
2. Dipartimento di Fisica, Univerisità di Roma Tor Vergata, via della Ricerca Scientifica 1, I-00133 Roma, Italy
3. INAF-Istituto di Radioastronomia, via Gobetti 101, I-40129 Bologna, Italy
4. Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Turing Building, Oxford Road, Manchester M13 9PL, UK
5. Department of Astronomy, University of Maryland, College Park, MD 20742, USA
6. X-ray Astrophysics Laboratory, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
7. INAF - Osservatorio Astronomico di Roma, Via Frascati 33, I-00044 Monte Porzio Catone (Roma), Italy