OBSERVATIONS OF GRB 990123 BY THE COMPT ON GAMMA RAY OBSERV AT ORY

GRB 990123 was the —rst 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 detec- tors. We —nd that gamma-ray —uxes are not correlated with the simultaneous optical observations and that the gamma-ray spectra cannot be extrapolated simply to the optical —uxes. The burst is well —tted 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 —rst intensity spike, 1470 ^ 110 lf l E p , keV, and then falls to D300 keV during the tail of the burst. The high-energy spectrum above D 1M eV is consistent with a power law with a photon index of about (3. By —uence, GRB 990123 is brighter than all but 0.4% of the GRBs observed with BATSE, clearly placing it on the power-law portion ( 3 2 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 "1.61 and assuming iso- ( 3 2 tropic emission, the gamma-ray energy exceeds 1054 ergs. Subject headings: gamma rays: burstsX-rays: bursts