[1] Univ Calif Irvine, Dept Stat, Irvine, CA 92617 USA
[2] Univ Calif Davis, Dept Stat, Davis, CA 95616 USA
[3] Univ London Imperial Coll Sci Technol & Med, Dept Math, Stat Sect, London SW7 2AZ, England
[4] Smithsonian Astrophys Observ, Div High Energy Astrophys, Cambridge, MA 02138 USA
[5] Univ Penn, Dept Phys, Philadelphia, PA 19104 USA
[6] NASA GSFC, Heliophys Sci Div, Greenbelt, MD 20771 USA