1. Xinjiang Astronomical Observatory, CAS, 150, Science 1-street, Urumqi, Xinjiang 830011, P. R. China
2. University of Chinese Academy of Science, 19A Yuquan Road, Beijing 100049, P. R. China
3. Key Laboratory of Radio Astronomy, Chinese Academy of Science, 830011 Urumqi, P. R. China
4. Netherlands Institute for Radio Astronomy, NL-7991 PD Dwingeloo, The Netherlands
5. Department of Solid State Physics and Nonlinear Physics, Faculty of Physics and Technology, Al-Farabi Kazakh National University, Almaty 050040, Kazakhstan