Multiwavelength observations of GRB 050820A: An exceptionally energetic event followed from start to finish

We present observations of the unusually bright and long gamma-ray burst GRB 050820A, one of the best sampled broadband data sets in the Swift era. The gamma-ray light curve is marked by a soft precursor pulse some 200 s before the main event; the lack of any intervening emission suggests that it is due to a physical mechanism distinct from the GRB itself. The large time lag between the precursor and the main emission enabled simultaneous observations in the gamma-ray, X-ray, and optical bandpasses, something only achieved for a handful of events to date. While the contemporaneous X-rays are the low-energy tail of the prompt emission, the optical does not directly track the gamma-ray flux. Instead, the early-time optical data appear consistent with the forward shock synchrotron peak passing through the optical and are therefore likely the beginning of the afterglow. On hour timescales after the burst, the X-ray and optical light curves are inconsistent with an adiabatic expansion of the shock into the surrounding region, but rather indicate that there is a period of energy injection. Observations at late times allow us to constrain the collimation angle of the relativistic outflow to 6.degrees 8 less than or similar to theta less than or similar to 9.degrees 3. Our estimates of both the kinetic energy of the afterglow (E-KE = 5.2(-4.1)(+7.9) x 10(51) ergs) and the prompt gamma-ray energy release (E-KE = 7.5(-4.1)(+7.9) x 10(51) ergs) make GRB 050820A one of the most energetic events for which such values could be determined.