Limits on Galactic Gamma-Ray Lines at 4.44 MeV and 6.13 MeV from Nuclear De-Excitation
Abstract
Inelastic collisions of low-energy cosmic-ray protons and alpha particles with interstellar 12C and 16O produce relatively narrow de-excitation lines at 4.44 and 6.13 MeV (FWHM ∼100 keV); impacts of cosmic-ray C and 16O nuclei on interstellar H atoms produce broad lines at these energies (FWHM ≃ 1 MeV). Such broad-line emission, at a level 5 × 10-5 γ (cm2 s)-1 at 4.44 MeV and ≃ 3 × 10-5 yγ(cm-2 s)-1 at 6.13 MeV, was recently reported from the Orion Nebula by the COMPTEL instrument aboard the Compton Gamma Ray Observatory. We have used over 9 years of data from the Gamma Ray Spectrometer experiment on the Solar Maximum Mission satellite to search for both narrow and broad lines from the directions of the Galactic center and anticenter, with 12C* and 16O* line intensities fixed in the ratios expected from theory and observed by COMPTEL No emission was detected in either broad or narrow lines from any source. Our 3 σ upper limits on the combined emission from 12C* and 16O* are 8.7 × 10-5 γ (cm2 s rad)-1 for narrow lines from the central radian of the Galaxy, 3.0 × 10-4 γ (cm2 s rad)-1 for broad lines from the same direction, and 3.0 × 10-4 γ (cm2 s)-1 for broad lines from a point source in the direction of Orion. Our results suggest that the Galactic distribution of Orion-like sources does not follow the expected dependence on the square of the CO abundance and, in particular, that the 12C and 16O abundances in the cosmic rays accelerated in Orion-like sources are not proportional to the ambient CO abundance. We also place constraints on the fraction of Galactic 26Al which can be due to cosmic-ray spallation reactions.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 1995
- DOI:
- 10.1086/175948
- Bibcode:
- 1995ApJ...448..157H
- Keywords:
-
- GALAXY: CENTER;
- GAMMA RAYS: OBSERVATIONS;
- ISM: ABUNDANCES;
- ISM: COSMIC RAYS