When Do Internal Shocks End and External Shocks Begin? Early‐Time Broadband Modeling of GRB 051111

Even with the renaissance in gamma-ray burst (GRB) research fostered by theSwift satellite, few bursts have both contemporaneous observations at longwavelengths and exquisite observations at later times across theelectromagnetic spectrum. We present here contemporaneous imaging with the KAITrobotic optical telescope, dense optical sampling with Lulin, and supplementedwith infrared data from PAIRITEL and radio to gamma-ray data from theliterature. For the first time, we can test the constancy of microphysicalparameters in the internal-external shock paradigm and carefully trace the flowof energy from the GRB to the surrounding medium. KAIT data taken <~1 minuteafter the start of GRB051111 and coinciding with the fading gamma-ray tail ofthe prompt emission indicate a smooth re-injection of energy into the shock. Nocolor change is apparent in observations beginning ~1.5 minutes after the GRBand lasting for the first hour after the burst. There are achromatic fluxmodulations about the best-fit model at late (t~10^4 s) times, possibly due tovariations in the external density. We find that the host-galaxy extinction iswell fit by a curve similar to that ofthe Small Magellanic Cloud. Low visualextinction, A_V~0.2 mag, combined with high column densities determined fromthe X-ray and optical spectroscopy (N_H> 10^21 cm^-2), indicate a lowdust-to-metals ratio and a possible over-abundance of the light metals. Anapparent small ratio of total to selective extinction (R_V~2) argues againstdust destruction by the GRB. Time constancy of both the IR/optical/UV spectralenergy distribution and the soft X-ray absorption suggests that the absorbingmaterial is not local to the GRB.Comment: 21 pages, 6 figures, to Appear in ApJ v653, December, 10 200