1. Chandra X-ray Centre, Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02143, USA
2. Department of Astronomy, Yale University, New Haven, CT 06511, USA
3. Department of Physics, Yale University, New Haven, CT 06520, USA
4. Department of Physics, University of Texas – Dallas, Richardson, TX 75080, USA
5. School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK