1. Senior scientist at the Laboratory for Astronomy and Solar Physics, NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771
2. Chief scientist at the Laboratory for High Energy Astrophysics, NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771
3. Assistant professor of astronomy at the Center for Astrophysics, Harvard College Observatory, Smithsonian Astrophysical Observatory, Cambridge, Massachusetts 02138
4. Professor in the Physics Department, Stanford University, Stanford, California 94305