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We present a comprehensive analysis of a bright, long duration (T90 ~ 257 s)GRB 110205A at redshift z= 2.22. The optical prompt emission was detected bySwift/UVOT, ROTSE-IIIb and BOOTES telescopes when the GRB was still radiatingin the gamma-ray band. Nearly 200 s of observations were obtainedsimultaneously from optical, X-ray to gamma-ray, which makes it one of theexceptional cases to study the broadband spectral energy distribution across 6orders of magnitude in energy during the prompt emission phase. By fitting thetime resolved prompt spectra, we clearly identify, for the first time, aninteresting two-break energy spectrum, roughly consistent with the standard GRBsynchrotron emission model in the fast cooling regime. Although the promptoptical emission is brighter than the extrapolation of the best fit X/gamma-rayspectra, it traces the gamma-ray light curve shape, suggesting a relation tothe prompt high energy emission. The synchrotron + SSC scenario is disfavoredby the data, but the models invoking a pair of internal shocks or having twoemission regions can interpret the data well. Shortly after prompt emission (~1100 s), a bright (R = 14.0) optical emission hump with very steep rise (alpha~ 5.5) was observed which we interpret as the emission from the reverse shock.It is the first time that the rising phase of a reverse shock component hasbeen closely observed. The full optical and X-ray afterglow lightcurves can beinterpreted within the standard reverse shock (RS) + forward shock (FS) model.In general, the high quality prompt emission and afterglow data allow us toapply the standard fireball shock model to extract valuable information aboutthe GRB including the radiation mechanism, radius of prompt emission R, initialLorentz factor of the outflow, the composition of the ejecta, as well as thecollimation angle and the total energy budget.

Panchromatic observations of the textbook GRB 110205A: constraining physical mechanisms of prompt emission and afterglow