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Gamma-ray bursts are produced as a result of cataclysmic events such as thecollapse of a massive star or the merger of two neutron stars. We monitored theposition of the close-by gamma-ray burst GRB~190829A, which originated from amassive star collapse, through very long baseline interferometry (VLBI)observations with the EVN and the VLBA, involving a total of 30 telescopesacross 4 continents. We carried out a total of 9 observations between 9 and 117days after the gamma-ray burst at 5 and 15 GHz, with a typical resolution offew milliarcseconds (mas). We obtained limits on the source size and expansionrate. The limits are in agreement with the size evolution entailed by adetailed modelling of the multi-wavelength light curves with a forward plusreverse shock model, which agrees with the observations across almost 18 ordersof magnitude in frequency (including the High Energy Stereoscopic System dataat TeV photon energies) and more than 4 orders of magnitude in time. Thanks tothe broad, high-cadence coverage of the afterglow, afterglow degeneracies arebroken to a large extent, allowing us to capture some unique physical insights:we find a low prompt emission efficiency $\lesssim 10^{-3}$; we constrain thefraction of electrons that are accelerated to relativistic speeds in theforward shock to be $\chi_e<13\%$ at the 90\% credible level; we find that themagnetic field energy density in the reverse shock downstream must decayrapidly after the shock crossing. While our model assumes an on-axis jet, ourVLBI astrometric measurements alone are not sufficiently tight as to excludeany off-axis viewing angle. On the other hand, we can firmly exclude the lineof sight to have been more than $2\,\mathrm{deg}$ away from the border of theregion that produced the prompt gamma-ray emission based on compactnessarguments.

Multi-wavelength view of the close-by GRB 190829A sheds light on gamma-ray burst physics