[1] Inst Fis Cantabria, CSIC, UC, Santander 39005, Spain
[2] Univ Leicester, Dept Phys & Astron, Leicester LE1 7RH, Leics, England
[3] Univ Cambridge, Inst Astron, Cambridge CB3 0HA, England
[4] Max Planck Inst Extraterr Phys, D-85741 Garching, Germany
[5] UCL, Mullard Space Sci Lab, Dorking RH5 6NT, Surrey, England
[6] Ctr Etud Spatiale Rayonnements, F-31028 Toulouse 04, France
[7] Astrophys Inst Potsdam, D-14482 Potsdam, Germany
[8] Osserv Astron Brera, I-20121 Milan, Italy
[9] Observ Astron, F-67000 Strasbourg, France
[10] Inst Astrofis Canarias, Tenerife 38200, Spain
[11] Univ Florence, Dipartimento Astron & Sci Spazio, I-50125 Florence, Italy
[12] Univ Bristol, Dept Phys, Bristol BS8 1TL, Avon, England