The early multicolor afterglow of GRB 050502a: Possible evidence for a uniform medium with density clumps

The 2 m robotic Liverpool Telescope reacted promptly to the gamma-ray burst GRB 050502a, discovered by INTEGRAL, and started observing 3 minutes after the onset of the burst. The automatic identification of a bright afterglow with r′ ∼ 15.8 mag triggered, for the first time, an observation sequence in the BVr′i′ filters during the first hour after a GRB. Observations continued for ∼1 day using the RoboNet-1.0 network of 2 m robotic telescopes. The light curve in all filters can be described by a simple power law with index of 1.2 ± 0.1. We find evidence for a bump rising at t ∼ 0.02 days in all filters. From the spectrum and the light curve, we investigate different scenarios and find possible evidence for a uniform circumburst medium with clumps in density, as in the case of GRB 021004. Other interpretations of such bumps, such as the effect of energy injection through refreshed shocks or the result of a variable energy profile, are less favored. The optical afterglow of GRB 050502a is likely to be the result of slow electron cooling, with the optical bands lying between the synchrotron peak frequency and the cooling frequency.