[1] Univ Amsterdam, Astron Inst Anton Pannekoek, NL-1098 SJ Amsterdam, Netherlands
[2] Ctr High Energy Astrophys, NL-1098 SJ Amsterdam, Netherlands
[3] Univ Palermo, Dipartimento Sci Fis & Astron, I-90123 Palermo, Italy
[4] SRON, Natl Inst Space Res, NL-3584 CA Utrecht, Netherlands
[5] Osserv Astron Roma, I-00040 Rome, Italy