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Anisotropic Broad Nuclear Gamma-Ray Lines: Application to the COMPTEL Observations of Orion

We investigated in detail the shape of the broad nuclear gamma-ray line emission resulting from accelerated C, N, and O interacting with ambient H and He in an attempt to fit the recent COMPTEL observations of Orion. The fact that the observed gamma-ray emission shows a strong enhancement in the 3-7 MeV region with significant structure within this energy band is the main motivation for assuming that the emission is due to nuclear de-excitations in C, N, and O. We found that the strong emission peak observed at ~4 MeV, with a red wing extending down to almost 3 MeV, can be understood as broad line emission produced by accelerated C, N, and O with a relatively hard spectrum but with the blue wing of the 12C line suppressed by the anisotropy of the interactions. The fact that the observed spectrum does not show narrow emission peaks at 4.44 and 6.13 MeV indicates that the bulk of the observed emission is produced by accelerated C, N, and O, rather than by accelerated protons and α-particles. The anisotropic interaction scenario removes the constraint placed by the line widths on the hardness of the accelerated ion spectrum, a useful feature because harder ion energy spectra are energetically more efficient than softer spectra. We have incorporated in our calculations the anisotropic angular distribution of the gamma-ray line emission in the rest frame of the heavy ions leading to line-splitting effects. The only feature in the COMPTEL data that would indicate the presence of splitting is a structure in the 5-7.5 MeV region. However, for calculations that account for the ~4 MeV peak, the characteristic maxima due to line splitting in this region are apparent only in models employing multiple sources.