1. Department of Astrophysics, University of Zurich , Winterthurerstrasse 190, CH-8057 Zürich , Switzerland
2. Department of Life and Physical Sciences, Fisk University , 1000 17th Avenue N., Nashville, TN 37208 , USA
3. Department of Physics & Astronomy, Vanderbilt University , 2301 Vanderbilt Place, Nashville, TN 37235 , USA
4. Physik-Institut, Universität Zürich , Winterthurerstrasse 190, CH-8057 Zürich , Switzerland
5. Max Planck Institute for Gravitational Physics (Albert Einstein Institute) , Am Mühlenberg 1, Potsdam D-14476 , Germany