1. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
2. National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100012, China
3. Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109-1107, USA
4. Tsinghua Center for Astrophysics and Department of Physics, Tsinghua University, Beijing 100084, China
5. Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
6. Department of Physics and Electronics, Rhodes Centre for Radio Astronomy Techniques & Technologies, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
7. Argelander-Institut für Astronomie, Auf dem Hügel 71, D-53121 Bonn, Germany
8. SKA South Africa Radio Astronomy Research Group, 3rd Floor, The Park, Park Road, Pinelands 7405, South Africa
9. National Centre for Radio Astrophysics, TIFR, Ganeshkhind, Pune 411007, India
10. Astronomisches Institut der Ruhr-Universität Bochum (AIRUB), Universitätsstrasse 150, D-44801 Bochum, Germany
11. International Centre for Radio Astronomy Research, M468, The University of Western Australia, Crawley, WA 6009, Australia
12. ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), WA 6009, Australia
13. Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), 44 Rosehill Street Redfern, NSW 2016, Australia
14. Purple Mountain Observatory, Chinese Academy of Sciences, 2 West-Beijing Road, Nanjing 210008, China