1. California Institute of Technology, TAPIR , Mailcode 350-17, Pasadena, CA 91125 , USA
2. Department of Space, Earth and Environment, Chalmers University of Technology , SE-412 93 Göteborg , Sweden
3. Department of Astrophysical Sciences, Princeton University , Princeton, NJ 08544 , USA
4. Department of Physics, Center for Astrophysics and Space Sciences, University of California San Diego , 9500 Gilman Drive, La Jolla, CA 92093 , USA
5. Department of Physics and Astronomy and CIERA , Northwestern University , 2145 Sheridan Road, Evanston, IL 60208 , USA
6. Black Hole Initiative, Harvard University , 20 Garden Street, Cambridge, MA 02138 , USA