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 burstGRB 050502a discovered by INTEGRAL and started observing 3 min after the onsetof the GRB. The automatic identification of a bright afterglow of r'~15.8triggered for the first time an observation sequence in the BVr'i' filtersduring the first hour after a GRB. Observations continued for ~1 day using theRoboNet-1.0 network of 2-m robotic telescopes. The light curve in all filterscan be described by a simple power law with index of 1.2 +/- 0.1. We findevidence for a bump rising at t~0.02 days in all filters. From the spectrum andthe light curve we investigate different interpretative scenarios and we findpossible evidence for a uniform circumburst medium with clumps in density, asin the case of GRB 021004. Other interpretations of such bumps, such as theeffect of energy injection through refreshed shocks or the result of a variableenergy profile, are less favored. The optical afterglow of GRB 050502a islikely to be the result of slow electron cooling with the optical bands lyingbetween the synchrotron peak frequency and the cooling frequency.