1. National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, China
2. School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China
3. Physics Department and Tsinghua Centre for Astrophysics, Tsinghua University, Beijing 100084, China
4. Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
5. Aix Marseille Univ, CNRS, CNES, LAM, UMR 7326, F-13388 Marseille 13, France
6. School of Physics and Astronomy, University Nottingham, Nottingham NG7 2RD, UK
7. Department of Physics and Astronomy, Haverford College, 370 Lancaster Avenue, Haverford, PA 19041, USA
8. Institute of Cosmology and Gravitation, University of Portsmouth, Dennis Sciama Building, Portsmouth PO1 3FX, UK