1. Department of Physics, Ulyanovsk State Pedagogical University named after I. N. Ulyanov, 100 years V. I. Lenin's Birthday Square, 4, 432700 Ulyanovsk, Russia
2. Astrophysics and Cosmology Research Unit, School of Mathematics, Statistics and Computer Science, University of KwaZulu-Natal, P.B. X54 001, Durban 4000, South Africa