1. Jodrell Bank Centre for Astrophysics, University of Manchester , Oxford Road, Manchester M13 9PL, UK
2. School of Mathematics, Statistics and Physics, Newcastle University , Newcastle upon Tyne NE1 7RU, UK
3. Department of Physics, University of Pretoria , Lynnwood Road, Hatfield, Pretoria 0083, South Africa
4. National Institute for Theoretical and Computational Sciences (NITheCS) , 1 Jan Smuts Ave, Braamfontein, Johannesburg 2000, South Africa
5. Jodrell Bank Observatory , Lower Withington, Bomish Ln, Macclesfield SK11 9DW, UK
6. Wits Centre for Astrophysics, School of Physics, University of the Witwatersrand , Private Bag 3, Johannesburg 2050, South Africa
7. Instituto de Astrofísica de Andalucía (IAA , CSIC), Glorieta de las Astronomía, s/n, E-18008 Granada, Spain
8. Western Sydney University , Locked Bag 1797, Penrith South DC, NSW 2751, Australia
9. CSIRO Space and Astronomy, Australia Telescope National Facility , PO Box 76, Epping, NSW 1710, Australia
10. Dominion Radio Astrophysical Observatory , 1 Observatory Crescent, Ottawa, ON K1A 0C6, Canada