Observations of Grb 990123 by the Compt on Gamma Ray Observ at Ory

GRB 990123 was the Ðrst burst from which simultaneous optical, X-ray, and gamma-ray 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 Compton Gamma Ray Observatory detectors. We Ðnd that gamma-ray Ñuxes are not correlated with the simultaneous optical observations and that the gamma-ray spectra cannot be extrapolated simply to the optical Ñuxes. The burst is well Ðtted by the standard four-parameter GRB function, with the exception that excess emission compared with this function is observed below D15 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 spectrum, reaches an unusually high value during the Ðrst intensity spike, 1470 ^ 110 lfl Ep, keV, and then falls to D300 keV during the tail of the burst. The high-energy spectrum above D1 MeV is consistent with a power law with a photon index of about [3. By Ñuence, GRB 990123 is brighter than all but 0.4% of the GRBs observed with BATSE, clearly placing it on the power-law portion [ 32 of the intensity distribution. However, the redshift measured for the afterglow is inconsistent with the Euclidean interpretation of the power law. Using the redshift value of o1.61 and assuming iso[ 32 tropic emission, the gamma-ray energy exceeds 1054 ergs. Subject headings : gamma rays : bursts È X-rays : bursts