1. Department of Astronomy, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA
2. Department of Physics and Astronomy, Amherst College, C025 New Science Center, 25 East Dr., Amherst, MA 01002-5000, USA
3. Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, D-85748 Garching, Germany
4. X-Ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
5. Space Science Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA
6. Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France
7. National Space Institute, Technical University of Denmark, Elektrovej 327-328, DK-2800 Lyngby, Denmark
8. Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, P-00-478 Warszawa, Poland
9. Observatory and Erlangen Centre for Astroparticle Physics, Sternwartstr. 7, D-96049 Bamberg, Germany
10. South African Astronomical Observatory, PO Box 9, Observatory, Cape Town 7935, South Africa
11. Department of Astronomy, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa