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G.H. Share, R.J. Murphy, J.E. Grove, J.D. Kurfess (NRL), S.M. Matz (NWU)
We describe temporal and spectroscopic gamma-ray observations of GRB990123 by the Oriented Scintillation Spectrometer Experiment (OSSE) on NASA's COMPTON GAMMA RAY OBSERVATORY. The burst occurred well outside the field of view of the detectors but some of its radiation penetrated the extensive shielding and permitted spectroscopic observations >0.8 MeV. The shields surrounding the detectors recorded a peak rate of ~125,000 s-1 \ge125 keV. The burst is highly structured, with some features rising or falling on time-scales as short as ~16 ms. Emission above a few hundred keV primarily occurred during two peaks covering ~25 s while emission at lower energies continued for an additional ~45 s. We measured the incident spectrum >0.8 MeV during the two peaks in the detector best-positioned to observe the burst. Emission was observed up to ~5 MeV. The incident spectrum can be fit with a power law having a flux of 1.64 ± 0.24 \gamma cm-2 s-1 MeV-1 at 1 MeV and an exponent of -2.78 ± 0.17 (errors are 1-sigma statistical; there is probably an additional 20% systematic error because the source is well outside the aperture). Integrating the energy under this power law, we obtain a fluence of (1.10 ± 0.33) \times 10-4 erg cm-2 from 0.8 to 9 MeV.
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