The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. J.C.A.M.-J. is the recipient of an Australian Research Council Future Fellowship (FT140101082) funded by the Australian Government. A.J.T. is supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Post-Graduate Doctoral Scholarship (PGSD2-490318-2016). A.J.T. and G.R.S. acknowledge support from NSERC Discovery Grants (RGPIN-402752-2011 and RGPIN-06569-2016). M.J.M. acknowledges support from a Science and Technology Facilities Council (STFC) Ernest Rutherford Fellowship. D.A. acknowledges support from the Royal Society. G.E.A. is the recipient of an Australian Research Council Discovery Early Career Researcher Award (project number DE180100346) funded by the Australian Government. T.M.B. acknowledges a financial contribution from the agreement ASI-INAF n.2017-14-H.0. P.G.J. acknowledges funding from the European Research Council under ERC Consolidator Grant agreement 647208. S. Markoff and T.D.R. acknowledge support from a Netherlands Organisation for Scientific Research (NWO) Veni Fellowship and Vici Grant, respectively. K.P.M. acknowledges support from the Oxford Centre for Astrophysical Surveys, which is funded through the Hintze Family Charitable Foundation. K.P.M. is currently a Jansky Fellow of the National Radio Astronomy Observatory. This work profited from discussions carried out during a meeting on multi-wavelength rapid variability organized at the International Space Science Institute (ISSI) Beijing by T.M.B. and D. Bhattacharya. The authors acknowledge the worldwide effort in observing this outburst, and the planning tools (created by T. Marsh and coordinated by C. Knigge) that enabled these observations.

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Nature thanks José L. Gómez and Christopher Reynolds for their contribution to the peer review of this work.