PANCHROMATIC OBSERVATIONS OF THE TEXTBOOK GRB 110205A: CONSTRAINING PHYSICAL MECHANISMS OF PROMPT EMISSION AND AFTERGLOW

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 by Swift/UVOT, ROTSE-IIIb, and BOOTES telescopes when the gamma-ray burst (GRB) was still radiating in the γ-ray band, with optical light curve showing correlation with γ-ray data. Nearly 200 s of observations were obtained simultaneously from optical, X-ray, to γ-ray (1 eV to 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 (∼1100 s), a bright (R = 14.0) optical emission hump with very steep rise (α ∼ 5.5) was observed, which we interpret as the reverse shock (RS) emission. It is the first time that the rising phase of an RS component has been closely observed. The full optical and X-ray afterglow light curves 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 (RGRB ∼ 3 × 1013 cm), initial Lorentz factor of the outflow (Γ0 ∼ 250), the composition of the ejecta (mildly magnetized), the collimation angle, and the total energy budget.