[1] Keele Univ, Sch Phys & Geog Sci, Astrophys Grp, Keele ST5 5BG, Staffs, England
[2] Univ Maryland Baltimore Cty, Ctr Space Sci & Technol, 1000 Hilltop Circle, Baltimore, MD 21250 USA
[3] Osserv Astron Brera, INAF, Via Bianchi 46, I-23807 Merate, LC, Italy
[4] NASA, Goddard Space Flight Ctr, Xray Astrophys Lab, Greenbelt, MD 20771 USA
[5] Univ Maryland, Dept Astron, College Pk, MD 20742 USA
[6] Univ Maryland, CRESST, College Pk, MD 20742 USA