1. Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany
2. Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
3. Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305-4085, USA
4. Department of Physics, Stanford University, 452 Lomita Mall, Stanford, CA 94305-4085, USA
5. Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada
6. Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM 87131, USA
7. European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany