1. Università dell'Insubria, Via Valleggio 11, I-22100 Como, Italy
2. INAF – Osservatorio astronomico di Brera, Via Bianchi 46, I-23807 Merate (LC), Italy
3. APC, Astroparticules et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10, rue Alice Domon et Léonie Duquet, F-75205 Paris Cedex 13, France
4. INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, I-40129 Bologna, Italy
5. University of Oxford, Department of Physics, Astrophysics, Keble Road, Oxford OX1 3RH, UK
6. Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 106, Ensenada B. C. 22800, Mexico