  The 2-m robotic Liverpool Telescope reacted promptly to the gamma-ray burst
GRB 050502a discovered by INTEGRAL and started observing 3 min after the onset
of the GRB. The automatic identification of a bright afterglow of r'~15.8
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 interpretative scenarios and we 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.
