1. Department of Physics and Electronics, Rhodes University , Artillery Road, Grahamstown, 6140 , South Africa
2. South African Radio Astronomy Observatory , FIR Street, Observatory, 7925, Cape Town , South Africa
3. INAF – Istituto di Radioastronomia , Via Piero Gobetti 101, I-40129 Bologna , Italy
4. Dipartimento di Fisica e Astronomia, Università di Bologna , Via Piero Gobetti 93/2, I-40129 Bologna , Italy
5. INAF – Osservatorio Astronomico di Cagliari , Via della Scienza 5, I-09047 Selargius (CA) , Italy
6. ASTRON, the Netherlands Institute for Radio Astronomy , Postbus 2, NL-7990 AA Dwingeloo , the Netherlands