A. K. We, M. Keith, B. Coles, and G. Hobbs, Part of this work is based on observations with the 100-m telescope of the Max-Planck-Institut für Radioastronomie (MPIfR) at Effelsberg. The Nançay Radio Observatory is operated by the Paris Observatory, associated with the French Centre National de la Recherche Scientifique (CNRS) We acknowledge financial support from 'Programme National de Cosmologie and Galaxies' (PNCG) of CNRS/INSU, France. Pulsar research at the Jodrell Bank Centre for Astrophysics and the observations using the Lovell Telescope is supported by a consolidated grant from the STFC in the UK. The Westerbork Synthesis Radio Telescope is operated by the Netherlands Institute for Radio Astronomy (ASTRON) with support from The Netherlands Foundation for Scientific Research NWO. RNC acknowledges the support of the International Max Planck Research School Bonn/Cologne and the Bonn-Cologne Graduate School. KJL gratefully acknowledge support from National Basic Research Program of China, 973 Program, 2015CB857101 and NSFC 11373011. PL acknowledges the support of the International Max Planck Research School Bonn/Cologne. SO is supported by the Alexander von Humboldt Foundation. JWTH acknowledges funding from an NWO Vidi fellowship and from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Starting Grant agreement no, CMFM was supported by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme. AS is supported by the Royal Society This research was in part supported by ST's appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory

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