) Chinese Journal ofAstronomy and Astrophysics The Early (<1 hr) Multi–Colour Afterglow of GRB 050502a with the 2-m Liverpool Telescope

Abstract The 2-m robotic Liverpool Telescope automatically discovered the optical after-glow of the INTEGRAL gamma-ray burst GRB 050502a 3 min after the GRB onset. The automatic identification of a bright optical transient of r0 15:8 triggered for the first time a multi–colour observation sequence in the BV r0i0 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 in-terpretation of the bump, as the result of a refreshed shock, appears to be more problematic, although it cannot be ruled out either