1. Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK
2. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
3. Shanghai Astronomical Observatory, 80 Nandan Road, 200030 Shanghai, China
4. Department of Astronomy, School of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
5. Key Laboratory for Particle Astrophysics and Cosmology (MOE) / Shanghai Key Laboratory for Particle Physics and Cosmology, Shanghai 200240, China