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We present observations of the unusually bright and long gamma-ray burst GRB050820A, one of the best-sampled broadband data sets in the Swift era. Thegamma-ray light curve is marked by a soft precursor pulse some 200 s before themain event; the lack of any intervening emission suggests that it is due to aphysical mechanism distinct from the GRB itself. The large time lag between theprecursor and the main emission enabled simultaneous observations in thegamma-ray, X-ray, and optical band-passes, something only achieved for ahandful of events to date. While the contemporaneous X-rays are the low-energytail of the prompt emission, the optical does not directly track the gamma-rayflux. Instead, the early-time optical data appear mostly consistent with theforward shock synchrotron peak passing through the optical, and are thereforelikely the beginning of the afterglow. On hour time scales after the burst, theX-ray and optical light curves are inconsistent with an adiabatic expansion ofthe shock into the surrounding region, but rather indicate that there is aperiod of energy injection. Observations at late times allow us to constrainthe collimation angle of the relativistic outflow to theta = 6.8 - 9.3 degrees.Our estimates of both the kinetic energy of the afterglow and the promptgamma-ray energy release make GRB 050820A one of the most energetic events forwhich such values could be determined.

Multiwavelength Observations of GRB 050820A: An Exceptionally Energetic Event Followed from Start to Finish