ON THE FERMI LARGE AREA TELESCOPE SURPLUS OF DIFFUSE GALACTIC GAMMA-RAY EMISSION

Recent observations of diffuse Galactic gamma-ray emission (DGE) by the Fermi Large Area Telescope (Fermi-LAT) have shown significant deviations, above a few GeV to about 100 GeV, from DGE models that use the GALPROP code for the propagation of cosmic ray (CR) particles outside their sources in the Galaxy and their interaction with the target distributions of the interstellar gas and radiation fields. The surplus of radiation observed is most pronounced in the inner Galaxy, where the concentration of CR sources is strongest. The present study investigates this "Fermi-LAT Galactic Plane Surplus" by estimating the. gamma-ray emission from the sources themselves, which is disregarded in the above DGE models. It is shown that the expected hard spectrum of CRs, still confined in their sources (source cosmic rays, SCRs), can indeed explain this surplus. The method is based on earlier studies regarding the so-called EGRET GeV excess, which by now is generally interpreted as an instrumental effect. The contribution from SCRs is also predicted to increasingly exceed the DGE models above 100 GeV, up to gamma-ray energies of about 10 TeV, where the corresponding surplus exceeds the hadronic part of the DGE by about one order of magnitude. Above such energies, the emission surplus should decrease again with energy due to the finite lifetime of the assumed supernova remnant sources. Observations of the DGE in the inner Galaxy at 15 TeV with the ground-based Milagro gamma-ray detector and, at TeV energies, with the ARGO-YBJ detector are interpreted to provide confirmation of a significant SCR contribution to the DGE.