Probing the Cosmic Ray density in the inner Galaxy

TL;DR

This study uses 11 years of Fermi-LAT gamma-ray data to measure cosmic-ray densities near molecular clouds in the inner Galaxy, finding they are consistent with local values and attributing the observed gradient to cosmic-ray sources.

Contribution

It provides the first detailed point-by-point measurement of cosmic-ray density in the inner Galaxy using gamma-ray observations, clarifying the origin of the gamma-ray emission peak.

Findings

Cosmic-ray densities near molecular clouds match local measurements.

The gamma-ray emission peak is due to cosmic-ray accelerators, not a global propagation change.

The cosmic-ray density gradient is linked to the distribution of accelerators.

Abstract

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.