TITLE: GCN GRB OBSERVATION REPORT NUMBER: 231 SUBJECT: GRB990123, OSSE observations DATE: 99/01/27 09:57:44 GMT FROM: Steven M. Matz at Northwestern U/OSSE S. M. Matz (Northwestern University), G. H. Share, R. Murphy, and J. D. Kurfess (Naval Research Lab) report on behalf of the OSSE team: The OSSE instrument on board the Compton Gamma-Ray Observatory detected strong emission from GRB 990123 in its central detectors and in its active NaI shields. While the burst was not in the detectors' field of view, it was still detected to >3 MeV in two 16 s intervals. The rise and first strong peak (at about trigger+25 s) are roughly (within 1--2 s) aligned at all energies. The low-energy (<100 keV) burst emission appears to last significantly longer than the higher energy emission. Detailed spectral analysis is underway; preliminary analysis indicates that the average spectrum of the main burst can be well described above 0.8 MeV by a photon power law with an index of about -3. Time histories from the OSSE central detectors in different energy ranges from 50 keV to >3 MeV are posted on the OSSE burst web page ( http://www.astro.nwu.edu/astro/osse/bursts/ ) along with a preliminary spectrum of the burst. Also available are a plot and raw data for the high time resolution (16 ms) history of the first 60 s of the burst from the triggered OSSE shield data (>100 keV). This report may be cited.