1. INAF – IASF Palermo , Via U. La Malfa 153, I-90146 Palermo, Italy
2. Università degli Studi di Palermo , Dipartimento di Fisica e Chimica, via Archirafi 36, I-90123 Palermo, Italy
3. MIT Kavli Institute for Astrophysics and Space Research , Cambridge, MA 02139, USA
4. Cahill Center for Astronomy and Astrophysics, California Institute of Technology , Pasadena, CA 91125, USA
5. Dr. Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics , Sternwartstr. 7, D-96049 Bamberg, Germany
6. Institute of Astronomy , Madingley Road, CB3 0HA Cambridge, UK
7. Université de Toulouse , CNRS, IRAP, 9 Avenue du colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France
8. European Space Agency (ESA) , European Space Astronomy Centre (ESAC), E-28691 Villanueva de la Canada, Madrid, Spain