Constraints on the cosmic rays in the Small Magellanic Cloud

Observations of the Small Magellanic Cloud using the Energetic Gamma Ray Experiment Telescope on the Compton Observatory yield an upper limit (95% confidence) for gamma-ray emission >100 MeV of 0.5×{}10^-7 photon/cm² s. The expected flux if the cosmic rays are universal rather than galactic in origin, is (2.4-0.5)×{}10^-7 photon/cm² s, only a third of which arise from cosmic ray electron interactions. Hence, the bulk of the cosmic ray energy density is almost certainly not metagalactic, and therefore galactic in origin.