1. Department of Physics ‘A. Pontremoli’, University of Milan, via G. Celoria 16, I-20133 Milan, Italy
2. INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via A. Corti 12, I-20133 Milan, Italy
3. Janusz Gil Institute of Astronomy, University of Zielona Góra, ul Szafrana 2, PL-65-265 Zielona Góra, Poland
4. INAF – Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monteporzio Catone, Italy
5. Scuola Universitaria Superiore IUSS Pavia, Piazza della Vittoria 15, I-27100 Pavia, Italy
6. INFN, Sezione di Pavia, via A. Bassi 6, I-27100 Pavia, Italy
7. Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, CA 95064, USA
8. Department of Physics, Laboratory for Space Research, University of Hong Kong, Pokfulam Road, Hong Kong