Limits on Nuclear Gamma‐Ray Emission from Orion

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Abstract
The discovery of γ-ray line emission in the 3-7 MeV range from the Orion complex was recently reported. The observed Compton Telescope (COMPTEL) spectrum suggested that the emission results from the de-excitation of excited states of 12C and 16O. We report on a search for these lines using the Oriented Scintillation Spectroscopy Experiment (OSSE) on the Compton Gamma Ray Observatory (CGRO) during a 5 week observation in 1995 from April to June. The OSSE detectors were pointed midway between the Orion A and B radio sources in three different viewing configurations. We find no compelling evidence for line emission near 4.4 or 6.1 MeV. The sensitivity of the OSSE measurements is dependent on the widths of the reported C and O lines and on the source location and spatial extent. A point source at the flux level reported by COMPTEL and located on-axis would have been detected by OSSE at ~7 σ and ~5 σ levels of confidence for narrow and broad lines, respectively. A spatially distributed source of the same strength with a distribution following the intense CO emission localized around Orion A and Orion B would have been detected by OSSE at ~3.5 σ and ~2.5 σ, respectively. Thus, these OSSE observations require that any γ-ray line source must be even more extended to be consistent with the reported COMPTEL intensity.