Cosmic rays in the inner galaxy and the diffusion properties of the interstellar medium

Recent measurements of cosmic gamma ray intensities up to TeV energies have been used to estimate the spectral shape of the parent cosmic ray particles present in the interstellar medium. The case is made for the particle spectrum in the Inner Galaxy being flatter than locally and in the Outer Galaxy. Of various possible explanations we make the case for the propagation of the particles being different in the more turbulent interstellar medium of the Inner Galaxy. The characteristic parameter α for the so called ‘anomalous diffusion’ is expected to be less in the Inner Galaxy than that locally and the corresponding power law spectral exponent of the cosmic ray particles γ will differ from that locally. Arguments are presented favouring a value of α   lower than the local one by ΔαΔα = 0.2; the consequence is that Δγ∼0.1Δγ∼0.1 for the parent particles.

► Diffuse gamma rays in the Inner Galaxy have a flatter energy spectrum than locally. ► More supernovae in the Inner Galaxy lead to a more turbulent interstellar medium. ► Propagation in the turbulent medium leads to a flatter cosmic ray energy spectrum.