[1] Univ Calif San Diego, Ctr Astrophys & Space Sci, La Jolla, CA 92093 USA
[2] Ctr Etud Saclay, Serv Astrophys, F-91191 Gif Sur Yvette, France
[3] Univ Paris 07, F-91191 Gif Sur Yvette, France
[4] Univ Amsterdam, Astron Inst Anton Pannekoek, NL-1098 SJ Amsterdam, Netherlands
[5] Ctr High Energy Astrophys, NL-1098 SJ Amsterdam, Netherlands
[6] MIT, Dept Phys, Cambridge, MA 02139 USA
[7] MIT, Space Res Ctr, Cambridge, MA 02139 USA
[8] Univ Utrecht, Astron Inst, NL-3508 TA Utrecht, Netherlands
[9] CSIRO, Australia Telescope Natl Facil, Epping, NSW 1710, Australia
[10] Harvard Smithsonian Ctr Astrophys, Cambridge, MA 02138 USA