1. Department of Physics, Università di Pisa, I-56127 Pisa, Italy
2. Sezione di Pisa, INFN, I-56127 Pisa, Italy
3. Université Lyon, Université Claude Bernard Lyon 1, CNRS, IP2I Lyon/IN2P3, UMR 5822, F-69622 Villeurbanne, France
4. European Gravitational Observatory (EGO), Cascina, I-56021 Pisa, Italy