Probing the Cosmic-Ray Density in the Inner Galaxy
Abstract
The galactic diffuse γ-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 γ-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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 2021
- DOI:
- 10.3847/2041-8213/abcaa9
- arXiv:
- arXiv:2101.09510
- Bibcode:
- 2021ApJ...907L..11P
- Keywords:
-
- Gamma-rays;
- Interstellar medium;
- Giant molecular clouds;
- Cosmic rays;
- 637;
- 847;
- 653;
- 329;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- ApJL 907 L11 (2021)