[1] Univ Hong Kong, Dept Phys, Hong Kong, Hong Kong, Peoples R China
[2] Univ Hong Kong, Ctr Theoret & Computat Phys, Hong Kong, Hong Kong, Peoples R China
[3] Univ Szeged, Dept Expt Phys, H-6720 Szeged, Hungary
[4] Max Planck Inst Radioastron, D-53121 Bonn, Germany
[5] Univ Portsmouth, Inst Cosmol & Gravitat, Portsmouth PO1 2EG, Hants, England
[6] Univ Lisbon, Ctr Astron & Astrofis, P-1749016 Lisbon, Portugal