1. Astrophysics, Department of Physics, University of Oxford , Keble Road, Oxford OX1 3RH, UK
2. Department of Astronomy, University of Cape Town , Private Bag X3, Rondebosch 7701, South Africa
3. National Radio Astronomy Observatory , Socorro, NM 87801, USA
4. Cahill Center for Astronomy and Astrophysics , MC 249-17 California Institute of Technology, Pasadena, CA 91125, USA
5. Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Brera , via E. Bianchi 46, I-23807 Merate (LC), Italy
6. Department of Physics and Astronomy, Northwestern University , Evanston, IL 60208, USA
7. MERLIN/VLBI National Facility, Jodrell Bank Observatory, The University of Manchester , Macclesfield, Cheshire SK11 9DL, UK
8. International Centre for Radio Astronomy Research-Curtin University , Perth, WA 6845, Australia
9. Cluster for Pioneering Research, Institute of Physical and Chemical Research (RIKEN) , 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
10. Okayama Observatory, Kyoto University , 3037-5 Honjo, Kamogatacho, Asakuchi, Okayama 719-0232, Japan
11. Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo , 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
12. Department of Astronomy, Graduate School of Science, Kyoto University , Oiwakecho, Kitashirakawa, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan