OSSE Observations of GX 339-4

The Oriented Scintillation Spectrometer Experiment on the Compton Gamma Ray Observatory observed the Galactic black hole candidate GX 339-4 as a target of opportunity in 1991 September, in response to the outburst reported by BATSE. We report here on energy spectra in the 50 keV to 10 MeV range obtained by OSSE. The source was detected from 50 to 400 keV at a level relative to the Crab nebula of 30. The observed spectrum was described reasonably well by a power law with an exponential cutoff a least-squares fit yielded a photon index of .88 plus or minus .05 and a cutoff energy of 68 plus or minus 2 keV. The addition of a Compton reflection component did not significantly improve the overall fit. An optically-thin thermal bremsstrahlung spectrum also provides a good fit, and the thermal Comptonization model of Sunyaev and Titarchuk, while deficient in describing the data above about 200 keV, cannot formally be ruled out. A pure power law with reflection does not fit the observed spectrum. During a follow-up observation made in 1991 November, the intensity of the source below 100 keV had dropped by more than a factor of 40, and it was no longer detected above about 100 keV.