  Gamma-ray bursts (GRBs) are thought to result from the interaction of an
extremely relativistic outflow interacting with a small amount of material
surrounding the site of the explosion. Multi-wavelength observations covering
the gamma-ray to radio wavebands allow investigations of this `fireball' model.
On 23 January 1999 optical emission was detected while the gamma-ray burst was
still underway. Here we report the results of gamma-ray, optical/infra-red,
sub-mm, mm and radio observations of this burst and its afterglow, which
indicate that the prompt and afterglow emissions from GRB 990123 are associated
with three distinct regions in the fireball. The afterglow one day after the
burst has a much lower peak frequency than those of previous bursts; this
explains the short-lived nature of the radio emission, which is not expected to
reappear. We suggest that such differences reflect variations in the
magnetic-field strengths in the afterglow emitting regions.
