  The Swift burst GRB 110205A was a very bright burst visible in the Northern
hemisphere. GRB 110205A was intrinsically long and very energetic and it
occurred in a low-density interstellar medium environment, leading to delayed
afterglow emission and a clear temporal separation of the main emitting
components: prompt emission, reverse shock, and forward shock. Our observations
show several remarkable features of GRB 110205A : the detection of prompt
optical emission strongly correlated with the BAT light curve, with no temporal
lag between the two ; the absence of correlation of the X-ray emission compared
to the optical and high energy gamma-ray ones during the prompt phase ; and a
large optical re-brightening after the end of the prompt phase, that we
interpret as a signature of the reverse shock. Beyond the pedagogical value
offered by the excellent multi-wavelength coverage of a GRB with temporally
separated radiating components, we discuss several questions raised by our
observations: the nature of the prompt optical emission and the spectral
evolution of the prompt emission at high-energies (from 0.5 keV to 150 keV) ;
the origin of an X-ray flare at the beginning of the forward shock; and the
modeling of the afterglow, including the reverse shock, in the framework of the
classical fireball model.
