We thank the Australia Telescope National Facility (ATNF) engineering and technical staff for their help in supporting these observations, and especially thank the staff of the Murchison Radio-astronomy observatory. We thank C. Flynn, P. Edwards, N. Tejos and V. McIntyre for comments on the manuscript, and members of the Commensal Real-time ASKAP Fast Transients (CRAFT) team for discussions. We thank the Murchison Widefield Array (MWA) principal engineer, R. Wayth, for access to the Galaxy supercomputer graphics processing units (GPU) cluster. R.M.S. and S.O. acknowledge Australian Research Council (ARC) grant FL150100148. R.M.S. also acknowledges support through ARC grant CE170100004. G.G. acknowledges support through a Commonwealth Scientific and Industrial Research Organisation (CSIRO) Office of the Chief Executive (OCE) postdoctoral fellowship. Parts of this research were conducted by the ARC Centre of Excellence for All-Sky Astrophysics (CAASTRO; grant CE110001020). This research was also supported by the ARC through grant DP18010085. The Australian SKA Pathfinder and Parkes radio telescopes are part of the ATNF, which is managed by the CSIRO. Operation of ASKAP is funded by the Australian Government with support from the National Collaborative Research Infrastructure Strategy. ASKAP uses the resources of the Pawsey Supercomputing Centre. Establishment of ASKAP, the Murchison Radio-astronomy Observatory and the Pawsey Supercomputing Centre are initiatives of the Australian Government, with support from the Government of Western Australia and the Science and Industry Endowment Fund. We acknowledge the Wajarri Yamatji people as the traditional owners of the Observatory site. This research has made use of the National Aeronautics and Space Administration (NASA)/Infrared Processing and Analysis Center (IPAC) Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.

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Nature thanks J. Cordes, D. Lorimer and S. Ransom for their contribution to the peer review of this work.