We are grateful to the VLBA, VLA and GBT staff, especially M. Claussen, A. Mioduszewski, T. Minter, F. Ghigo, W. Brisken, K. O’Neill and M. McKinnon, for their support with the HSA observations. We thank V. Dhawan and P. Demorest for help with observational issues with the VLBI system at the VLA. K.P.M. thanks A. Mioduszewski, E. Momjian, E. Greisen, T. Pearson and S. Kulkarni for discussions. We thank M. Kasliwal for providing comments on the manuscript. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities. K.P.M. is currently a Jansky Fellow of the National Radio Astronomy Observatory. K.P.M. acknowledges support from the Oxford Centre for Astrophysical Surveys, which is funded through the Hintze Family Charitable Foundation, for some initial work presented here. E.N. acknowledges the support of an ERC starting grant (GRB/SN) and an ISF grant (1277/13). A.T.D. is the recipient of an Australian Research Council Future Fellowship (FT150100415). G.H. acknowledges the support of NSF award AST-1654815. A.H. acknowledges support by the I-Core Program of the Planning and Budgeting Committee and the Israel Science Foundation. A.C. acknowledges support from the NSF CAREER award number 1455090 titled ‘CAREER: Radio and gravitational-wave emission from the largest explosions since the Big Bang’.

Author contributions

K.P.M., A.T.D., S.B., G.H. and D.A.F. coordinated the VLBI observations. A.T.D. and K.P.M. performed the VLBI data processing. O.G. and E.N. carried out the theoretical study, including analytic calculations and numerical simulations, with some input from K.H. K.P.M., A.T.D., E.N., G.H. and D.A.F. wrote the paper. A.C. and A.H. compiled the references. A.H., A.D. and K.P.M. compiled Methods. O.G., A.T.D., A.H. and K.P.M. prepared the figures. All co-authors discussed the results and provided comments on the manuscript.