Abstract The 2-m robotic Liverpool Telescope automatically discovered the optical afterglow of the INTEGRAL gamma-ray burst GRB 050502a 3 min after the GRB onset. The automatic identification of a bright optical transient of r'~15.8 triggered for the first time a multi–colour observation sequence in the BVr'i' filters during the first hour after a GRB. All the four light curves are fitted by a simple power law with index of 1.2±0.1. We also find evidence for an achromatic bump rising at t~0.02 days. We investigate different scenarios compatible with the data. We find possible evidence for a uniform circumburst medium with clumps in density, as in the case of GRB 021004. The alternative case of a wind environment cannot be ruled out, although it can hardly account for our observations. The alternative interpretation of the bump, as the result of a refreshed shock, appears to be more problematic, although it cannot be ruled out either.
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