1. Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands
2. Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ 08544, USA
3. Mathematical Sciences and STAG Research Centre, University of Southampton, SO17 1BJ Southampton, UK
4. Helmholtz-Zentrum Dresden-Rossendorf, POB 51 01 19, D-01314 Dresden, Germany
5. Department of Astronomy, University of Maryland, College Park, MD 20742, USA