The contribution of Galactic TeV pulsar wind nebulae to Fermi large area telescope diffuse emission

The large-scale diffuse gamma - ray flux observed by Fermi Large Area Telescope (Fermi-LAT) in the 1-100 GeV energy range, parameterized as proportional to E-Gamma, has a spectral index Gamma that depends on the distance from the Galactic center. This feature, if attributed to the diffuse emission produced by cosmic rays interactions with the interstellar gas, can be interpreted as the evidence of a progressive cosmic ray spectral hardening towards the Galactic center. This interpretation challenges the paradigm of uniform cosmic rays diffusion throughout the Galaxy. We report on the implications of TeV Pulsar Wind Nebulae observed by the High Energy Stereoscopic System (H.E.S.S.) Galactic Plane Survey in the 1-100 TeV energy range for the interpretation of Fermi-LAT data. We argue that a relevant fraction of this population cannot be resolved by Fermi-LAT in the GeV domain providing a relevant contribution to the large-scale diffuse emission, ranging within similar to 4%-40% of the total diffuse y-ray emission in the inner Galaxy. This additional component may account for a large part of the spectral index variation observed by Fermi-LAT, weakening the evidence of cosmic ray spectral hardening in the inner Galaxy.

Automated summary

The large-scale diffuse gamma-ray flux observed by Fermi-LAT shows a spectral index variation near the Galactic center, suggesting a progressive cosmic ray spectral hardening towards the center. The study of TeV Pulsar Wind Nebulae by H.E.S.S. provides evidence that a fraction of this population contributes significantly to the large-scale diffuse emission. This additional component weakens the evidence of cosmic ray spectral hardening observed by Fermi-LAT.