1. Astrophysics Research Center of the Open University (ARCO), The Open University of Israel 1 , P.O Box 808, Ra'anana 4353701, Israel
2. Department of Natural Sciences, The Open University of Israel 2 , P.O Box 808, Ra'anana 4353701, Israel
3. Department of Physics, The George Washington University 3 , Washington, DC 20052, USA