The large-scale diffuse $\gamma-$ray flux observed by Fermi-LAT in the 1-100 GeV energy range is parameterized as a power law with spectral index $\Gamma$ that depends on the distance from the Galactic center.
This feature, if attributed to the diffuse emission produced by cosmic rays (CR) interactions with the interstellar gas, can be interpreted as the evidence of a progressive CR spectral hardening towards the Galactic center, challenging the standard cosmic rays diffusion paradigm.
We report on the implications of TeV Pulsar Wind Nebulae observed by the HESS Galactic Plane Survey in the 1-100 TeV energy range for the interpretation of Fermi-LAT data.
We show that the consistency among HGPS and Fermi-LAT (3FGL) catalogue requires that Galactic sources, observed by both experiments, have a spectral break at $\sim 0.1-1.0$ TeV and an harder emission spectrum in the GeV domain.
Such spectral shape is well compatible with the expected gamma-ray emission in young PWNe due to Inverse Compton scattering of high-energy electrons on background radiation fields.
We argue that a fraction of this population cannot be resolved by Fermi-LAT in the GeV domain providing a relevant contribution to the large-scale diffuse emission, viz. the $30\%$ of the total diffuse $\gamma$-ray emission in the inner Galaxy.
The inclusion of this additional component in fitting Fermi-LAT data strongly affects the spectral index for the truly diffuse emission, that can be increased by an amount $\Delta \Gamma=0.17$, weakening the evidence of a progressive hardening of the cosmic-ray proton spectrum in the inner Galaxy.
|Reference to paper (DOI or arXiv)