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.

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Cited by in corpus (11)

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• Unresolved sources naturally contribute to PeV $\gamma$-ray diffuse emission observed by Tibet AS$\gamma$
• Cosmic-Ray Transport in Varying Galactic Environments
• The contribution of Galactic TeV pulsar wind nebulae to Fermi-LAT diffuse emission
• Probing the galactic cosmic-ray density with current and future $\gamma$-ray instruments
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• Absorption Line Observations of H${}_3^{+}$ and CO in Sight Lines Toward the Vela and W28 Supernova Remnants
• A new method of reconstructing Galactic three-dimensional structures using ultralong-wavelength radio observations
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• Magnetic field spreading from stellar and galactic dynamos into the exterior