[1] Harvard Smithsonian Ctr Astrophys, Cambridge, MA 02138 USA
[2] USN, Res Lab, Div Space Sci, Washington, DC 20375 USA
[3] Arizona State Univ, Sch Earth & Space Explorat, Tempe, AZ 85287 USA
[4] Osservatorio Astron Palermo GS Vaiana, I-90134 Palermo, Italy
[5] Univ Leicester, Dept Phys & Astron, Leicester LE1 7RH, Leics, England
[6] Liverpool John Moores Univ, Astrophys Res Inst, Birkenhead CH41 1LD, Merseyside, England
[7] Keele Univ, Astrophys Grp, Keele ST5 5BG, Staffs, England
[8] Univ Cent Lancashire, Ctr Astrophys, Preston PR1 2HE, Lancs, England
[9] Univ Minnesota, Dept Astron, Sch Phys & Astron, Minneapolis, MN 55455 USA
[10] ESAC, XMM Sci Operat Ctr, Madrid 28080, Spain
[11] Landessternwarte Konigstuhl, D-69117 Heidelberg, Germany
[12] Univ Manchester, Jodrell Bank Observ, Sch Phys & Astron, Macclesfield SK11 9DL, Cheshire, England
[13] W Chester Univ, Dept Geol & Astron, W Chester, PA 19383 USA