[1] Univ Southampton, Phys & Astron, Southampton SO17 1BJ, Hants, England
[2] Inst Astrofis Canarias IAC, Via Lactea S-N, E-38205 San Cristobal la Laguna, Sc De Tenerife, Spain
[3] Univ La Laguna, Dept Astrofis, Via Lactea S-N, E-38205 San Cristobal la Laguna, Sc De Tenerife, Spain
[4] Univ Groningen, Kapteyn Astron Inst, POB 800, NL-9700 AV Groningen, Netherlands
[5] RIKEN Cluster Pioneering Res, Extreme Nat Phenomena RIKEN Hakubi Res Team, 2-1 Hirosawa, Wako, Saitama 3510198, Japan
[6] Eureka Sci Inc, 2452 Delmer St, Oakland, CA 94602 USA
[7] SRON Netherlands Inst Space Res, SRON, Sorbonnelaan 2, NL-3584 CA Utrecht, Netherlands
[8] MIT, Kavli Inst Astrophys & Space Res, 70 Vassar St, Cambridge, MA 02139 USA
[9] Univ Amsterdam, Anton Pannekoek Inst Astron, Sci Pk 904, NL-1098 XH Amsterdam, Netherlands
[10] NASA, Goddard Space Flight Ctr, Astrophys Sci Div, Greenbelt, MD 20771 USA
[11] Univ Cagliari, Dipartimento Fis, SP Monserrato Sestu Km 0-7, I-09042 Monserrato, Italy
[12] Harvard Smithsonian Ctr Astrophys, 60 Garden St, Cambridge, MA 02138 USA
[13] Villanova Univ, Dept Phys, Villanova, PA 19085 USA
[14] Univ Roma Tor Vergata, Dept Phys, Via Ric Sci 1, I-00133 Rome, Italy
[15] Univ Maryland, Dept Astron, College Pk, MD 20742 USA
[16] INAF Osservatorio Astron Roma, Via Frascati 33, I-00078 Rome, Italy