[1] Harvard Smithsonian Ctr Astrophys, Cambridge, MA 02138 USA
[2] NASA, George C Marshall Space Flight Ctr, Huntsville, AL 35805 USA
[3] Univ Space Res Assoc, NSSTC, Huntsville, AL 35805 USA
[4] Stanford Univ, Kavli Inst Particle Astrophys & Cosmol, Stanford, CA 94309 USA
[5] Natl Radio Astron Observ, Socorro, NM 87801 USA
[6] Ben Gurion Univ Negev, Dept Phys, IL-84105 Beer Sheva, Israel
[7] Univ Amsterdam, Astron Inst Anton Pannekoek, NL-1098 SJ Amsterdam, Netherlands
[8] Inst Adv Study, Princeton, NJ 08540 USA
[9] Univ Sydney, Sch Phys, Sydney, NSW 2006, Australia
[10] Univ Manchester, Jodrell Bank Observ, Macclesfield SK11 9DL, Cheshire, England
[11] Univ Southampton, Sch Phys & Astron, Southampton SO17 1BJ, Hants, England
[12] Joint Inst VLBI Europe, NL-7990 AA Dwingeloo, Netherlands
[13] CSIRO, Australia Telescope Natl Facil, Epping, NSW 1710, Australia
[14] Los Alamos Natl Lab, Los Alamos, NM 87545 USA
[15] NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA