Observation of a strong gamma-ray burst on the spacelab 2 mission

A strong, confirmed gamma-ray burst was observed by a background-monitoring scintillation detector on the Spacelab 2 mission. The peak of the burst was at 00:56:38 UT on August 5, 1985. The large size of the detector allowed observations up to 16 MeV with high efficiency. A high data rate provided time-resolved observations over the energy range from 60 keV to 16 MeV, limited only by counting statistics.The burst was dominated by a single peak, ∼2 s wide, with softer, lower-level emission lasting ∼20 s> after the main peak. There was no evidence for time structure less than ∼0.2 s anywhere in the burst in any energy range. These characteristics are similar to a sizeable fraction (∼25%) of burst seen in the Konus catalog and we suggest that they are distinct from the more complex, “spiky” bursts and may have a different emission mechanism.In the energy range from ∼560 keV to ∼10 meV, the burst peaks ∼0.3 s before the peak at lower energies. Radiation in the energy range ∼10 to ∼16 MeV was detected at a confidence level of >96%, about 3 s before the lower energy radiation with roughly the same pulse width. This radiation is not detected during the main part of the burst. The energy of this burst in the range above 1 MeV is a significant fraction of the total burst energy, confirming the earlier SMM results.