[1] Monash Univ, Sch Phys & Astron, Monash Ctr Astrophys, Clayton, Vic 3800, Australia
[2] Aarhus Univ, Dept Phys & Astron, Ny Munkegade 120, DK-8000 Aarhus C, Denmark
[3] Tsung Dao Lee Inst, Shanghai 200240, Peoples R China
[4] Shanghai Jiao Tong Univ, Sch Phys & Astron, Dept Astron, Shanghai 200240, Peoples R China
[5] Univ Minnesota, Sch Phys & Astron, Minneapolis, MN 55455 USA
[6] Swinburne Univ Technol, OzGrav, Hawthorn, Vic 3122, Australia
[7] Queens Univ Belfast, Sch Math & Phys, Astrophys Res Ctr, Belfast BT7 1NN, Antrim, North Ireland
[8] Univ Bonn, Argelander Inst Astron, Hugel 71, D-53121 Bonn, Germany
[9] Max Planck Inst Radioastron, Hugel 69, D-53121 Bonn, Germany