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GRB 990123 was the first burst from which simultaneous optical, X-ray and gammaray emission was detected; its afterglow has been followed by an extensive set of radio, optical and X-ray observations. We have studied the gamma-ray burst itself as observed by the CGRO detectors. We find that gamma-ray fluxes are not correlated with the simultaneous optical observations, and the gamma-ray spectra cannot be extrapolated simply to the optical fluxes. The burst is well fit by the standard four-parameter GRB function, with the exception that excess emission compared to this function is observed below ∼ 15 keV during some time intervals. The burst is characterized by the typical hard-to-soft and hardness-intensity correlation spectral evolution patterns. The energy of the peak of the νf ν spectrum, E p , reaches an unusually high value during the first intensity spike, 1470 ± 110 keV, and then falls to ∼300 keV during the tail of the burst. The high-energy spectrum above ∼ 1 MeV is consistent with a power law with a photon index of about −3. By fluence, GRB 990123 is brighter than all but 0.4% of the GRBs observed with BATSE, clearly placing it on the −3/2 power-law portion of the intensity distribution. However, the redshift measured for the afterglow is inconsistent with the Euclidean interpretation of the −3/2 power-law. Using the redshift value of ≥ 1.61 and assuming isotropic emission, the gamma-ray fluence exceeds 10 54 ergs.

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