Low-Latitude Galactic γ -ray Emission: A Probe, not a Proof

Abstract

The emission of high energy (above 70 MeV) γ -rays from the galactic disc has been mapped by the COS-B mission with unprecedented detail. The results for |b| < 15 degrees are seen to contain evidence of structures correlated with the Galaxy on various scales, from the `grand design' down to granularities, showing that the diffuse interstellar medium, with its cosmic ray content, is well mapped by high energy γ -ray astronomy. Two new detailed correlations are proposed, one local and one in the medium-distance scale, to corroborate the above statement. After discussing the importance of the discrete, unresolved sources also discovered by COS-B, an astrophysical process is sketched suggesting a scenario for enhanced emission in regions where interstellar medium shocks can accelerate cosmic rays. Finally, the contribution of the nucleon and electron cosmic ray components in generating the galactic γ -rays in different energy ranges is briefly discussed, and seen to remain an open question.