1. INAF-Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Florence, Italy
2. Max-Planck-Institute for Extraterrestrial Physics, Garching D-85748, Germany
3. Ural Federal University, Ekaterinburg 620002, Russia
4. Engineering Research Institute, ‘Ventspils International Radio Astronomy Centre’ of Ventspils University of Applied Sciences, Inženieru 101, Ventspils LV-3601, Latvia
5. Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK