1. Departament de Física, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, E-08860 Castelldefels, Spain
2. Department of Physics, University of Warwick, Coventry CV4 7AL, UK
3. Institut d’Estudis Espacials de Catalunya, Ed. Nexus-201, c/Gran Capità 2-4, E-08034 Barcelona, Spain
4. Anton Pannekoek Institute for Astronomy, University of Amsterdam, NL-1090 GE Amsterdam, the Netherlands
5. Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599, USA