Vol 6, No S1

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

C. Guidorzi, A. Monfardini, A. Gomboc, C. G. Mundell, I. A. Steele, D. Carter, M. F. Bode, R. J. Smith, C. J. Mottram, M. J. Burgdorf, N. R. Tanvir, N. Masetti, E. Pian


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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