[1] Guizhou Normal Univ, Sch Phys & Elect Sci, Guiyang 550001, Peoples R China
[2] Guizhou Normal Univ, Guizhou Prov Key Lab Radio Astron & Data Proc, Guiyang 550001, Peoples R China
[3] Open Univ Israel, ARCO Astrophys Res Ctr, Dept Nat Sci, 1 Univ Rd,PO Box 808, Raanana IL-43537, Israel
[4] Max Planck Inst Astrophys, Karl Schwarzschild Str 1, Garching, Germany
[5] Univ Sussex, Astron Ctr, Dept Phys & Astron, Brighton BN19QH, England
[6] Univ Groningen, Kapteyn Astron Inst, PO BOX 800, NL-9700AV Groningen, Netherlands
[7] Stockholm Univ, Oskar Klein Ctr, Dept Astron, SE-10691 AlbaNova, Sweden
[8] Tel Aviv Univ, Sch Phys & Astron, Dept Astrophys, IL-69978 Tel Aviv, Israel