1. Centre for Theoretical Studies, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
2. Discipline of Astronomy, Astrophysics and Space Engineering, Indian Institute of Technology Indore, Simrol, Indore 453552, India
3. Department of Physics, Blackett Laboratory, Imperial College, London SW7 2AZ, UK
4. Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur 721302, India