1. Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803-4001, USA
2. Department of Physics, Texas Tech University, Box 41051, Lubbock, TX 79409-1051, USA
3. Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
4. SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
5. Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
6. Department of Astrophysics/ IMAPP, Radbound University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, the Netherlands
7. Department of Physics, University of Warwick, Coventry CV4 7AL, UK