The triple-peaked afterglow of GRB 210731A from X-ray to radio frequencies

Context. GRB 210731A was a long-duration (T90 = 22.5 s) gamma-ray burst discovered by the Burst Alert Telescope (BAT) aboard the Neil Gehrels Swift Observatory. Swift triggered the wide-field, robotic MeerLICHT optical telescope in Sutherland; it began observing the BAT error circle 286 s after the Swift trigger and discovered the optical afterglow of GRB 210731A in its first 60-s q-band exposure. Multi-colour observations of the afterglow with MeerLICHT revealed a light curve that showed three peaks of similar brightness within the first four hours. The unusual optical evolution prompted multi-wavelength follow-up observations that spanned from X-ray to radio frequencies. Aims. We present the results of our follow-up campaign and interpret our observations in the framework of the synchrotron forward shock model. Methods. We performed temporal and spectral fits to determine the spectral regime and external medium density profile, and performed detailed multi-wavelength theoretical modelling of the afterglow following the last optical peak at ∼0.2 days to determine the intrinsic blast wave parameters. Results. We find a preference for a stellar wind density profile consistent with a massive star origin, while our theoretical modelling results in fairly typical shock microphysics parameters. Based on the energy released in γ rays and the kinetic energy in the blast wave, we determine a low radiative efficiency of η ≈ 0.02. The first peak in the optical light curve is likely the onset of the afterglow. We find that energy injection into the forward shock offers the simplest explanation for the subsequent light curve evolution, and that the blast wave kinetic energy increasing by a factor of ∼1000 from the first peak to the last peak is indicative of substantial energy injection. Our highest-likelihood theoretical model over-predicts the 1.4 GHz flux by a factor of approximately three with respect to our upper limits, possibly implying a population of thermal electrons within the shocked region.