1. National Astronomical Observatories, Chinese Academy of Sciences, 20 Datun Road, Beijing 100020, China
2. Department of Physics and Astronomy, University of Kentucky, 505 Rose Street, Lexington, KY 40506, USA
3. Sub-Department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
4. Physics Department and Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, China
5. Jodrell Bank Centre for Astrophysics, Alan Turing Building, The University of Manchester, Manchester M13 9PL, UK