1. Physics and Astronomy, University of Kentucky , 505 Rose street, Lexington, KY 40506, USA
2. Center for Astrophysics | Harvard & Smithsonian , 60 Garden Street, Cambridge, MA 02138, USA
3. Max-Planck-Institut für Astronomie , K’onigstuhl 17, D-69117 Heidelberg, Germany
4. European Space Agency (ESA), European Space Research and Technology Centre (ESTEC) , Keplerlaan 1, NL-2201 AZ Noordwijk, the Netherlands
5. SRON Netherlands Institute for Space Research , Sorbonnelaan 2, NL-3584 CA Utrecht, the Netherlands
6. Instituto di Astrofisica Spaziale e Fisica Cosmica (INAF-IASF) , Milano, via A. Corti 12, I-20133 Milano, Italy
7. INAF - Osservatorio Astronomico di Trieste , via Tiepolo 11, I-34143 Trieste, Italy