1. Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, the Netherlands
2. Geneva Observatory, University of Geneva, Maillettes 51, 1290 Sauverny, Switzerland
3. LUPM, Université de Montpellier, CNRS, Place Eugène Bataillon, F-34095 Montpellier, France
4. Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
5. Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA
6. Department of Physics and Astronomy, Howard University, Washington, DC 20059, USA
7. Center for Research and Exploration in Space Science and Technology, and X-ray Astrophysics Laboratory, NASA/GSFC, Greenbelt, MD 20771, USA