tokens: ACKNOWLEDGEMENTS The Parkes Observatory is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by Commonwealth Scientific and Industrial Research Organisation. The Green Bank Observatory is a facility of the National Science Foundation (NSF) operated under cooperative agreement by Associated Universities, Inc. Pulsar research at the Jodrell Bank Centre for Astrophysics and the observations using the Lovell Telescope are supported by a consolidated grant from the Science and Technology Facilities Council in the UK. This work is also based on observations with the 100-m telescope of the Max-Planck-Institut für Radioastronomie at Effelsberg. This work is supported by the ARC Centres of Excellence CE110001020 (CAASTRO) and CE170100004 (OzGrav). Survey processing was conducted in association with CAASTRO at the Australian National Computational Infrastructure high-performance computing centre at the Australian National University. The authors wish to thank Marina Berezina, Eleni Graikou, Alex Kraus, and Laura Spitler for their assistance with observations at Effelsberg, along with Natalia Lewandowska and Ryan Lynch for their assistance with observations at the GBT. We also thank West Virginia University for its financial support of GBT operations, which enabled some of the observations for this project. ADC acknowledges the support of both the International Max Planck Research School for Astronomy and Astrophysics at the Universities of Bonn and Cologne, and the Bonn-Cologne Graduate School of Physics and Astronomy. PCCF and AR gratefully acknowledge financial support by the European Research Council (ERC) for the ERC Starting grant BEACON under contract No. 279702, and continued support from the Max Planck Society. MK, RK, and RPE gratefully acknowledge support from ERC Synergy Grant ‘BlackHoleCam’ Grant Agreement Number 610058. MAM was supported by NSF award number AST-1517003. DRL was supported by NSF award number OIA-1458952.
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