Probing the Cosmic Ray density in the inner Galaxy

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.

References in corpus (5)

• Physical properties of molecular clouds for the entire Milky Way disk
• Parametrization of gamma-ray production cross-sections for pp interactions in a broad proton energy range from the kinematic threshold to PeV energies
• Kinematic Distances to Molecular Clouds identified in the Galactic Ring Survey
• Gamma-ray sky points to radial gradients in cosmic-ray transport
• Precision Measurement of the Proton Flux in Primary Cosmic Rays from 1 GV to 1.8 TV with the Alpha Magnetic Spectrometer on the International Space Station

Cited by in corpus (3)

• Cosmic-Ray Transport in Varying Galactic Environments
• Absorption Line Observations of H${}_3^{+}$ and CO in Sight Lines Toward the Vela and W28 Supernova Remnants
• Search for enhanced TeV gamma-ray emission from Giant Molecular Clouds using H.E.S.S