The LOFAR cosmic ray key science project acknowledges funding from an Advanced Grant of the European Research Council (FP/2007–2013)/ERC Grant Agreement number 227610. The project has also received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 640130). We furthermore acknowledge financial support from FOM (FOM-project 12PR304). A.N. is supported by the DFG (research fellowship NE 2031/2-1). T.W. is supported by the DFG (research fellowship NE WI 4946/1-1). This paper is based (in part) on data obtained with the International LOFAR Telescope (ILT) under project code LC6_003. LOFAR25 is the Low Frequency Array designed and constructed by ASTRON. It has observing, data processing and data storage facilities in several countries, that are owned by various parties (each with their own funding sources), and that are collectively operated by the ILT foundation under a joint scientific policy. The ILT resources have benefited from the following recent major funding sources: CNRS-INSU, Observatoire de Paris and Université d’Orléans, France; BMBF, MIWF-NRW, MPG, Germany; Science Foundation Ireland (SFI), Department of Business, Enterprise and Innovation (DBEI), Ireland; NWO, The Netherlands; The Science and Technology Facilities Council, UK; Ministry of Science and Higher Education, Poland.

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Nature thanks E. Williams and the other anonymous reviewer(s) for their contribution to the peer review of this work.