1. Special Astrophysical Observatory, The Russian Academy of Sciences , Nizhnij Arkhyz, Karachai-Cherkessian Republic 369167 , Russia
2. Helmholtz-Institut für Strahlen- und Kernphysik, University of Bonn , Nussallee 14-16, D-53115 Bonn , Germany
3. Faculty of Mathematics and Physics, Astronomical Institute, Charles University in Prague , V Holešovičkách 2, CZ-180 00 Praha 8 , Czech Republic