[1] Chinese Acad Sci, Shanghai Astron Observ, 80 Nandan Rd, Shanghai 200030, Peoples R China
[2] Univ Chinese Acad Sci, 19A Yuquan Rd, Beijing 100049, Peoples R China
[3] Shanghai Jiao Tong Univ, Tsung Dao Lee Inst, Shengrong Rd 520, Shanghai 201210, Peoples R China
[4] Fudan Univ, Ctr Astron & Astrophys, Shanghai 200438, Peoples R China
[5] Fudan Univ, Dept Phys, Shanghai 200438, Peoples R China
[6] Los Alamos Natl Lab, Los Alamos, NM 87545 USA
[7] Shanghai Jiao Tong Univ, Sch Phys & Astron, 800 Dongchuan Rd, Shanghai 200240, Peoples R China
[8] Goethe Univ Frankfurt, Inst Theoret Phys, Max von Laue Str 1, D-60438 Frankfurt, Germany
[9] Chinese Acad Sci, Key Lab Radio Astron & Technol, A20 Datun Rd, Beijing 100101, Peoples R China
[10] Max Planck Inst Radioastron, Hugel 69, D-53121 Bonn, Germany
[11] Peking Univ, Kavli Inst Astron & Astrophys, Beijing 100871, Peoples R China
[12] Peking Univ, Sch Phys, Dept Astron, Beijing 100871, Peoples R China
[13] Polish Acad Sci, Nicolaus Copernicus Astron Ctr, Bartycka 18, PL-00716 Warsaw, Poland
[14] Max Planck Inst Kernphys, Saupfercheckweg 1, D-69117 Heidelberg, Germany