1. Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast, Belfast BT7 1NN, UK
2. School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
3. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21210, USA
4. Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21210, USA
5. Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
6. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA