GRB 221009A afterglow reduced visibilities

The high angular resolution (mas) of our observations allowed us, for the second time ever, after GRB 030329, to measure the projected size, s, of the relativistic shock caused by the expansion of the GRB ejecta into the surrounding medium.

Our observations support the expansion of the shock with a>4sigma-equivalent significance, and confirm its relativistic nature by revealing an apparently superluminal expansion rate. Fitting a power law expansion model, s~t^a^, to the observed size evolution, we find a slope a=0.69^+0.13^_-0.14_. Fitting the data at each frequency separately, we find different expansion rates, pointing to a frequency-dependent behaviour.