[1] Swales Aerosp, Beltsville, MD 20705 USA
[2] NASA, Goddard Space Flight Ctr, High Energy Astrophys Lab, Greenbelt, MD 20771 USA
[3] Univ Miami, Dept Phys, Coral Gables, FL 33146 USA
[4] Univ Wisconsin, Dept Phys, Madison, WI 53706 USA
[5] Yale Univ, Dept Phys, New Haven, CT 06520 USA