1. College of Physics, International Center of Future Science, Jilin University , Qianjin Street 2699, Changchun 130012 , P. R. China
2. Astronomical Observatory, Ivan Franko National University of Lviv , Kyryla i Methodia str. 8, Lviv 79005 , Ukraine
3. Faculty of Physics, Taras Shevchenko National University of Kyiv , 64/13, Volodymyrska str., 01601 Kyiv , Ukraine
4. Institute of Radio Astronomy of NASU , 4 Mystetstv str., 61002 Kharkiv , Ukraine