We present a comprehensive analysis of a bright , long duration ( T _ { 90 } \sim 257 s ) GRB 110205A at redshift z = 2.22 .
The optical prompt emission was detected by Swift /UVOT , ROTSE-IIIb and BOOTES telescopes when the GRB was still radiating in the \gamma -ray band , with optical lightcurve showing correlation with \gamma -ray data .
Nearly 200 s of observations were obtained simultaneously from optical , X-ray to \gamma -ray ( 1 eV - 5 MeV ) , which makes it one of the exceptional cases to study the broadband spectral energy distribution during the prompt emission phase .
In particular , we clearly identify , for the first time , an interesting two-break energy spectrum , roughly consistent with the standard synchrotron emission model in the fast cooling regime .
Shortly after prompt emission ( \sim 1100 s ) , a bright ( R = 14.0 ) optical emission hump with very steep rise ( \alpha \sim 5.5 ) was observed which we interpret as the the reverse shock emission .
It is the first time that the rising phase of a reverse shock component has been closely observed .
The full optical and X-ray afterglow lightcurves can be interpreted within the standard reverse shock ( RS ) + forward shock ( FS ) model .
In general , the high quality prompt and afterglow data allow us to apply the standard fireball model to extract valuable information including the radiation mechanism ( synchrotron ) , radius of prompt emission ( R _ { GRB } \sim 3 \times 10 ^ { 13 } cm ) , initial Lorentz factor of the outflow ( \Gamma _ { 0 } \sim 250 ) , the composition of the ejecta ( mildly magnetized ) , as well as the collimation angle and the total energy budget .