1. Department of Physics and Astronomy, University of Sheffield , Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK
2. Department of Physics, Florida State University , Tallahassee, FL 32306, USA
3. Astrophysics Research Institute, Liverpool John Moores University , 146 Brownlow Hill, Liverpool L3 5RF, UK
4. Department of Astronomy, University of California , Berkeley, CA 94720-3411, USA
5. Physics Department, Lancaster University , Lancaster LA1 4YB, UK
6. Department of Physics, University of Warwick , Gibbet Hill Road, Coventry CV4 7AL, UK
7. Department of Physics, University of Bath , Claverton Down, Bath BA2 7AY, UK
8. Center for Astrophysics and Cosmology, University of Nova Gorica , Vipavska 13, Nova Gorica 5000, Slovenia
9. Department of Physics and Earth Science, University of Ferrara , Via Saragat 1, I-44122 Ferrara, Italy
10. INFN -- Sezione di Ferrara , Via Saragat 1, 44122 Ferrara, Italy
11. INAF -- Osservatorio di Astrofisica e Scienza dello Spazio di Bologna , Via Piero Gobetti 101, 40129 Bologna, Italy