  The recent simultaneous detection of optical, X-ray and gamma-ray photons
from GRB990123 during the burst provides the first broadband multi-wavelength
characterization of the burst spectrum and evolution. Here we show that a
direct correlation exists between the time-varying gamma-ray spectral shape and
the prompt optical emission. This combined with the unique signatures of the
time-resolved spectra of GRB990123 convincingly supports earlier predictions of
the saturated Comptonization model. Contrary to other suggestions, we find that
the entire continuum from optical to gamma-rays can be generated from a single
source of leptons (electrons and pairs). The optical flux only appears to lag
the gamma-ray flux due to the high initial Thomson depth of the plasma. Once
the plasma has completely thinned out, the late time afterglow behavior of our
model is the same as in standard models based on the Blandford-McKee (1976)
solution.
