We thank Franz Kahn, who stimulated this work, for fruitful discussions. We also wish to thank Gregor Morfill for many helpful comments on the manuscript. DB is grateful to J. Trümper and the MPI für Extraterrestrische Physik, where most of this work was done, for excellent working conditions. He also thanks his colleagues Bruce Draine, Edward Jenkins and Russell Kulsrud at the Department of Astrophysical Sciences for interesting discussions, and Scott Tremaine and Princeton University, where this work was completed, for their hospitality. DB acknowledges support from the Deutsche Forschungsgemeinschaft (DFG) by a Heisenberg fellowship. The ROSAT project is supported by the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF/DLR) and the Max-Planck-Gesellschaft (MPG).
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001