[1] Univ Oxford, Dept Phys, Astrophys, Keble Rd, Oxford OX1 3RH, England
[2] Univ Cape Town, Dept Astron, Private Bag X3, ZA-7701 Rondebosch, South Africa
[3] Natl Radio Astron Observ, Socorro, NM 87801 USA
[4] CALTECH, Cahill Ctr Astron & Astrophys, MC 249-17, Pasadena, CA 91125 USA
[5] Ist Nazl Astrofis, Osservatorio Astron Brera, via E Bianchi 46, I-23807 Merate, LC, Italy
[6] North western Univ, Dept Phys & Astron, Evanston, IL 60208 USA
[7] Univ Manchester, MERLIN VLBI Natl Facil, Jodrell Bank Observ, Macclesfield SK11 9DL, Cheshire, England
[8] Curtin Univ, Int Ctr Radio Astron Res, Perth, WA 6845, Australia
[9] Inst Phys & Chem Res RIKEN, Cluster Pioneering Res, 2-1 Hirosawa, Saitama 3510198, Japan
[10] Kyoto Univ, Okayama Observ, 3037-5 Honjo,Kamogatacho, Asakuchi, Okayama 7190232, Japan
[11] Univ Tokyo, Grad Sch Arts & Sci, Dept Multidisciplinary Sci, 3-8-1 Komaba, Tokyo 1538902, Japan
[12] Kyoto Univ, Grad Sch Sci, Dept Astron, Sakyo Ku, Kyoto, Kyoto 6068502, Japan