1. Harvard-Smithsonian Center for Astrophysics; Harvard University, Cambridge, MA 02138, USA
2. Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA); Northwestern University, Evanston, IL 60208, USA
3. Department of Astronomy, Columbia University, New York, NY 10027, USA
4. School of Physics and Astronomy and Institute of Gravitational Wave Astronomy, University of Birmingham, Edgbaston B15 2TT, UK