During the first year of operation, 22 cosmic gamma-ray bursts were detected within the field of view of the COMPTEL instrument on board the Compton Gamma Ray Observatory. Spectra and time histories for the strongest of these bursts have been obtained from both the main instrument (0.75-30 MeV) and the burst modules (0.3-10 MeV). The deconvolved photon spectra for the majority of bursts are fit by a single power law model with spectral index between -1.6 and -2.8. One strong burst, GRB 910814, exhibited significant curvature and could not be fit by a single power law model. A broken power law model with a break in slope at ~ 2 MeV is a good fit to the time averaged spectrum of this burst. There is evidence in the COMPTEL data, at the 2.8σ level, for a changing break energy in GRB 910814, from above 2 MeV to below 1 MeV during the first 9 s of the burst. This is the first report of a time dependent turnover at MeV energies in the spectrum of a gamma-ray burst. GRB 910503 was the only burst in our sample which exhibited significant hard to soft evolution. We show that our observations of spectral evolution in the sample of data examined do not conflict with the assertion by SMM that hard to soft evolution is a characteristic feature of MeV emission from gamma-ray bursts. The strongest bursts were located using COMPTEL's independent imaging capability and location maps for each imaged burst are presented. The intersection of the COMPTEL locations with triangulation arcs derived from the Interplanetary Network (IPN) results in a smaller error box for counterpart searchers to examine.
Astronomy and Astrophysics
European Southern Observatory (ESO)
L.O. Hanlon, K. Bennett, W. Collmar, A. Connors, R. Diehl, R. van Dijk, J. Greiner, J.W. den Herder, W. Hermsen, R. M. Kippen, L. Kuiper, M. McConnell, J. Ryan, V. Schönfelder, H. Steinle, A.W. Strong, M. Varendorff, O.R. Williams, and C. Winkler. Observations of gamma-ray bursts by COMPTEL. 1994, Astron. Astrophys., 285, 161.