The radio afterglow of the ultra-long GRB 220627A

We present the discovery of the radio afterglow of the most distant ultra-long gamma-ray burst (GRB) detected to date, GRB~220627A at redshift $z=3.084$. Its prompt gamma-ray light curve shows a double-pulse profile, with the pulses separated by a period of quiescence lasting $\sim 15$,min, leading to early speculation it could be a strongly gravitationally lensed GRB. However, our analysis of the $\mathit{\text{Fermi}}$/GBM spectra taken during the time intervals of both pulses show clear differences in their spectral energy distributions, disfavouring the lensing scenario. We observed the radio afterglow from 7 to 456,d post-burst: an initial, steep decay ($F_{\nu }\propto t^{-2}$) is followed by a shallower decline ($F_{\nu }\propto t^{-1\mathrm{/}2}$) after $\sim 20$,d. There are three scenarios that could explain these radio properties: (i) energy injection from an additional, slower ejecta component catching up to the external shock; (ii) a stratified density profile going as $n\propto r^{-8\mathrm{/}3}$; or alternatively, (iii) the presence of a slow, wide ejecta component in addition to a fast, narrow ejecta component. We also conducted an independent test of the lensing hypothesis via Very Long Baseline Interferometry (VLBI) observations at $\sim 12$,d post-burst by searching, for the first time, for multiple images of the candidate lensed GRB afterglow. Our experiment highlighted the growing need for developments in real-time correlation capabilities for time-critical VLBI experiments, particularly as we advance towards the SKA and ngVLA era of radio astronomy.