[1] Princeton Univ Observ, Princeton, NJ 08544 USA
[2] Fermilab Natl Accelerator Lab, Batavia, IL 60510 USA
[3] Penn State Univ, Dept Astron & Astrophys, University Pk, PA 16802 USA
[4] Inst Adv Study, Princeton, NJ 08540 USA
[5] Pontificia Univ Catolica Chile, Fac Fis, Dept Astron & Astrofis, Santiago 22, Chile
[6] Johns Hopkins Univ, Dept Phys & Astron, Baltimore, MD 21218 USA
[7] Univ Chicago, Dept Astron & Astrophys, Chicago, IL 60637 USA
[8] Univ Chicago, Enrico Fermi Inst, Chicago, IL 60637 USA
[9] Univ Washington, Dept Astron, Seattle, WA 98195 USA
[10] Apache Point Observ, Sunspot, NM 88349 USA
[11] CALTECH, Dept Astron, Pasadena, CA 91125 USA
[12] Univ Tokyo, Inst Cosm Ray Res, Midori Ku, Tokyo 1888502, Japan
[13] USN Observ, Washington, DC 20392 USA
[14] USN Observ, Flagstaff Stn, Flagstaff, AZ 86002 USA
[15] Carnegie Mellon Univ, Dept Phys, Pittsburgh, PA 15232 USA
[16] Astrophys Inst Potsdam, D-14482 Potsdam, Germany