The role of unresolved PWNe to the gamma-ray diffuse emission at GeV

Abstract

We discuss the implications of the TeV galactic Pulsar Wind Nebulae (PWNe) population observed by HESS Galactic Plane Survey (HGPS) for the interpretation of Fermi-LAT data in the GeV domain. We show that consistency among HGPS and 3FGL catalogs requires that the average ratio $R_\Phi =\Phi_{\rm GeV}/\Phi_{\rm TeV}$, where $\Phi_{\rm GeV}$ ($\Phi_{\rm TeV}$) is the integrated gamma-ray flux emitted by sources in the range 1-100 GeV (1-100 TeV) probed by Fermi-LAT (HESS), is $R_\Phi = 500-1000$. Considering that the average spectral index of observed sources at TeV energies is $\beta_{\rm TeV} = 2.3$, the required value for $R_\Phi$ can be only obtained by assuming that sources have spectral break below 1 TeV and harder emission spectrum with $\beta_{\rm GeV} < 2$ in the GeV domain. Such spectral shapes are well compatible with the expected gamma-ray emission in young PWNe due to Inverse Compton scattering of high-energy electrons on background radiation fields. Finally, we show that a relevant fraction of the TeV source population cannot be resolved by Fermi-LAT in the GeV domain. We suggest that unresolved PWNe can provide a not negligible contribution to the large-scale diffuse emission observed by Fermi-LAT, possibly explaining the spectral hardening of this component toward the galactic center reported by Gaggero et al. 2018, Yang et al. 2016, Acero et al. 2016.