tokens: ACKNOWLEDGEMENTS PRB is supported by Track I award OIA-1458952 and is a member of the NANOGrav Physics Frontiers Center, which is supported by National Science Foundation award number 1430284. The Parkes radio telescope is part of the Australia Telescope National Facility which is funded by the Commonwealth of Australia for operation as a National Facility managed by the Commonwealth Scientific and Industrial Research Organisation. Data taken at Parkes is available via a public archive. We thank the referee Patrick Weltevrede for valuable comments that helped to improve the text.
ner_tags: O B-Person O O O B-Grant I-Grant I-Grant I-Grant O O O O O O B-Organization I-Organization I-Organization I-Organization O O O O B-Organization I-Organization I-Organization B-Grant I-Grant I-Grant O B-Telescope I-Telescope I-Telescope O O O O B-Telescope I-Telescope I-Telescope I-Telescope O O O O O B-Organization I-Organization I-Organization O O O O O O O O O B-Organization I-Organization I-Organization I-Organization I-Organization I-Organization O O O B-Telescope O O O O O O O O O O B-Person I-Person O O O O O O O O O