1. Department of Physics, University of Alabama in Huntsville, Huntsville, AL, 35899 USA
2. NASA National Space Science and Technology Center, Huntsville, AL, USA
3. University of Helsinki, FI-00014 Helsinki, Finland
4. CASA, Department of Astrophysical & Planetary Sciences, University of Colorado, Boulder, CO 80309, USA
5. Tartu Observatory, Observatooriumi 1, 61602 Tõravere, Estonia
6. Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany