1. Department of Physics and Astronomy ‘Augusto Righi’, University of Bologna , via Piero Gobetti 93/2, I-40129 Bologna , Italy
2. INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna , Via Gobetti 101, I-40129 Bologna , Italy
3. Department of Physics ‘Ettore Pancini’, University of Napoli Federico II , via Cinthia 9, I-80126 Napoli , Italy
4. INAF – Osservatorio Astronomico di Capodimonte , via Moiariello 16, I-80131 Napoli , Italy
5. INFN – Sezione di Napoli , via Cinthia 9, I-80126 Napoli , Italy
6. Chandra X-ray Center, Harvard-Smithsonian Center for Astrophysics , 60 Garden Street, Cambridge, MA 02138 , USA
7. Institute for Astronomy & Astrophysics, National Observatory of Athens , V. Paulou & I. Metaxa, Athens 11532 , Greece
8. Instituto de Física de Cantabria (CSIC-Universidad de Cantabria) , Avenida de los Castros, E-39005 Santander , Spain
9. Astrophysics Science Division, NASA Goddard Space Flight Center , Greenbelt, MD 20771 , USA