1. Istituto per le Applicazioni del Calcolo "M. Picone", CNR I-00161 Rome, Italy
2. International Center for Relativistic Astrophysics — I.C.R.A., University of Rome "La Sapienza", I-00185 Rome, Italy
3. INFN Sezione di Firenze, via G. Sansone, 1, I-50019 Sesto Fiorentino (FI), Italy
4. Department of Physics and Astronomy, University of Missouri-Columbia, Columbia, Missouri 65211, USA