Probing the Cosmic-Ray Density in the Inner Galaxy

The galactic diffuse gamma-ray emission, as seen by Fermi Large Area Telescope (LAT), shows a sharp peak in the region around 4 kpc from the Galactic center, which can be interpreted either as due to an enhanced density of cosmic-ray accelerators or to a modification of the particle diffusion in that region. Observations of gamma-rays originating in molecular clouds are a unique tool to infer the cosmic-ray density point by point, in distant regions of the Galaxy. We report here the analysis of 11 yr Fermi-LAT data, obtained in the direction of nine molecular clouds located in the 1.5-4.5 kpc region. The cosmic-ray density measured at the locations of these clouds is compatible with the locally measured one. We demonstrate that the cosmic-ray density gradient inferred from the diffuse gamma-ray emission is the result of the presence of cosmic-ray accelerators rather than a global change of the sea of Galactic cosmic rays due to their propagation.

Automated summary

Analyzing 11 years of Fermi-LAT data towards nine molecular clouds in the 1.5-4.5 kpc region, it is found that the cosmic-ray density measured at these cloud locations is consistent with locally measured values. The gradient of cosmic-ray density inferred from diffuse gamma-ray emission is due to the presence of cosmic-ray accelerators rather than a global change in the propagation of Galactic cosmic rays.