  The prompt optical emission of GRB 990123 was uncorrelated to the gamma-ray
light-curve and exhibited temporal properties similar to those of the
steeply-decaying, early X-ray emission observed by Swift at the end of many
bursts. These facts suggest that the optical counterpart of GRB 990123 was the
large-angle emission released during (the second pulse of) the burst. If the
optical and gamma-ray emissions of GRB 990123 have, indeed, the same origin
then their properties require that (i) the optical counterpart was synchrotron
emission and the gamma-rays arose from inverse-Compton scatterings
("synchrotron self-Compton model"), (ii) the peak-energy of the
optical-synchrotron component was at ~20 eV, and (iii) the burst emission was
produced by a relativistic outflow moving at Lorentz factor > 450 and at a
radius > 10^{15} cm, which is comparable to the outflow deceleration radius.
Because the spectrum of GRB 990123 was optically thin above 2 keV, the magnetic
field behind the shock must have decayed on a length-scale of <1% of the
thickness of the shocked gas, which corresponds to 10^6-10^7 plasma
skin-depths. Consistency of the optical counterpart decay rate and its spectral
slope (or that of the burst, if they represent different spectral components)
with the expectations for the large-angle burst emission represents the most
direct test of the unifying picture proposed here for GRB 990123.
