We thank W. J. G. de Blok, L. Connor, N. Maddox, E. Petroff, H. Vedantham and J. Weisberg for discussions. The European VLBI Network is a joint facility of independent European, African, Asian, and North American radio astronomy institutes. Scientific results from data presented in this publication are derived from the following EVN project code: EM135. This work was also based on simultaneous EVN and PSRIX data recording observations with the 100-m telescope of the Max-Planck-Institut für Radioastronomie at Effelsberg, and we thank the local staff for this arrangement. Our work is also based on observations obtained at the Gemini Observatory (programme DT-2019A-135), which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil). B.M. acknowledges support from the Spanish Ministerio de Economía y Competitividad (MINECO) under grants AYA2016-76012-C3-1-P and MDM-2014-0369 of ICCUB (Unidad de Excelencia “María de Maeztu”). J.W.T.H. 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 number 337062 (“DRAGNET”). M.B. is supported by an FRQNT Doctoral Research Award, Physics Department Excellence Award and a Mitacs Globalink Graduate Fellowship. R.K. is supported by ERC synergy grant number 610058 (“BlackHoleCam”). V.M.K. holds the Lorne Trottier Chair in Astrophysics and Cosmology, a Canada Research Chair and the R. Howard Webster Foundation Fellowship of CIFAR. V.M.K. receives support from an NSERC Discovery Grant and Herzberg Award, and from the FRQNT Centre de Recherche en Astrophysique du Québec. C.J.L. acknowledges support from NSF grant 1611606. D.M. is a Banting Fellow. K.A. acknowledges support from NSF grant AAG-1714897. B.A. is supported by a Chalk-Rowles Fellowship. A.M.A. acknowledges funding from an NWO Veni fellowship. S.B.-S. acknowledges support from NSF grant AAG-1714897. F.K. thanks the Swedish Research Council. U.-L.P. receives support from the Ontario Research Fund Research Excellence Program (ORF-RE), NSERC, the Simons Foundation, Thoth Technology Inc., and the Alexander von Humboldt Foundation. Z.P. is supported by a Schulich Graduate Fellowship. P.S. is a Dunlap Fellow and an NSERC Postdoctoral Fellow. The Dunlap Institute is funded through an endowment established by the David Dunlap family and the University of Toronto. K.M.S. is supported by an NSERC Discovery Grant, an Ontario Early Researcher Award, and a CIFAR fellowship. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The NANOGrav project receives support from National Science Foundation (NSF) Physics Frontiers Center award number 1430284. FRB research at the University of British Columbia (UBC) is supported by an NSERC Discovery Grant and by the Canadian Institute for Advanced Research. The CHIME/FRB baseband system is funded in part by a Canada Foundation for Innovation John R. Evans Leaders Fund award to I.H.S. The National Radio Astronomy Observatory (NRAO) is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Astronomical Image Processing System (AIPS) is a software package produced and maintained by NRAO. The Common Astronomy Software Applications (CASA) package is software produced and maintained by NRAO.