Observations of GRB 990123 by theCompton Gamma Ray Observatory

GRB 990123 was the first burst from which simultaneous optical, X-ray andgamma-ray emission was detected; its afterglow has been followed by anextensive set of radio, optical and X-ray observations. We have studied thegamma-ray burst itself as observed by the CGRO detectors. We find thatgamma-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 theexception that excess emission compared to this function is observed below ~15keV during some time intervals. The burst is characterized by the typicalhard-to-soft and hardness-intensity correlation spectral evolution patterns.The energy of the peak of the nu f_nu spectrum, E_p, reaches an unusually highvalue 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 ~MeV isconsistent with a power law with a photon index of about -3. By fluence, GRB990123 is brighter than all but 0.4% of the GRBs observed with BATSE, clearlyplacing it on the -3/2 power-law portion of the intensity distribution.However, the redshift measured for the afterglow is inconsistent with theEuclidean interpretation of the -3/2 power-law. Using the redshift value of >=1.61 and assuming isotropic emission, the gamma-ray fluence exceeds 10E54 ergs.Comment: Submitted to The Astrophysical Journal. 16 pages including 4 figure