1. Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia
2. PIFI Research Fellow, Key Laboratory for Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences,19B Yuquan Road, Beijing, 100049, China
3. Humboldt Research Fellow, Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
4. Astronomical Observatory, Volgina 7, 11000 Belgrade, Serbia
5. The National Radio Astronomy Observatory, 520 Edgemont Road Charlottesville, VA 2290, USA
6. Astrophysical Department, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
7. Faculty of Science, University of Kragujevac, Radoja Domanovića 12, 34000 Kragujevac, Serbia