1. Astrophysics Group, School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire ST5 5BG, UK
2. X-Ray and Observational Astronomy Group, Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
3. Center for Space Science and Technology, 1000 Hilltop Circle, University of Maryland, Baltimore County, Baltimore, MD 21250, USA