1. Physical Research Laboratory , Navrangpura, Ahmedabad 380 009, India
2. Indian Institute of Technology Gandhinagar, Palaj, Gandhinagar 382 355, India
3. Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research , Homi Bhabha Road, Mumbai 400 005, India
4. Purple Mountain Observatory, Chinese Academy of Sciences , Nanjing 210033, China