1. School of Physics and Astronomy, Sun Yat-Sen University, Guangzhou 510275, China
2. Institute of Astronomy, Madingley Road, Cambridge CB3 0HA
3. Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
4. Astrophysics Science Division, X-ray Astrophysics Laboratory, Code 662, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
5. INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, I-23807 Merate, Italy
6. Dipartimento di Fisica ‘G. Occhialini’, Università di Milano – Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy
7. Center for Astrophysics, Guangzhou University, Guangzhou 510006, China
8. Department of Astronomy, School of Physics, Peking University, Beijing 100871, China
9. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China