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

Abstract

Even with the renaissance in gamma-ray burst (GRB) research fostered by the\nSwift satellite, few bursts have both contemporaneous observations at long\nwavelengths and exquisite observations at later times across the\nelectromagnetic spectrum. We present here contemporaneous imaging with the KAIT\nrobotic optical telescope, dense optical sampling with Lulin, and supplemented\nwith infrared data from PAIRITEL and radio to gamma-ray data from the\nliterature. For the first time, we can test the constancy of microphysical\nparameters in the internal-external shock paradigm and carefully trace the flow\nof energy from the GRB to the surrounding medium. KAIT data taken <~1 minute\nafter the start of GRB051111 and coinciding with the fading gamma-ray tail of\nthe prompt emission indicate a smooth re-injection of energy into the shock. No\ncolor change is apparent in observations beginning ~1.5 minutes after the GRB\nand lasting for the first hour after the burst. There are achromatic flux\nmodulations about the best-fit model at late (t~10^4 s) times, possibly due to\nvariations in the external density. We find that the host-galaxy extinction is\nwell fit by a curve similar to that ofthe Small Magellanic Cloud. Low visual\nextinction, A_V~0.2 mag, combined with high column densities determined from\nthe X-ray and optical spectroscopy (N_H> 10^21 cm^-2), indicate a low\ndust-to-metals ratio and a possible over-abundance of the light metals. An\napparent small ratio of total to selective extinction (R_V~2) argues against\ndust destruction by the GRB. Time constancy of both the IR/optical/UV spectral\nenergy distribution and the soft X-ray absorption suggests that the absorbing\nmaterial is not local to the GRB.\n