The Fermi-LAT model of interstellar emission for standard point source analysis

Abstract

We describe the development of the model for interstellar gamma-ray emission that is the standard adopted by the LAT team and is publicly available. The model is based on a linear combination of templates for interstellar gas column density and for the inverse Compton emission. The spectral energy distributions of the gamma-ray emission associated with each template are determined from a fit to 4 years of Fermi-LAT data in 14 independent energy bins from 50 MeV to 50 GeV. We fit those distributions with a realistic model for the emission processes to extrapolate to higher energies. We also include large-scale structures like Loop I and the Fermi bubbles following an iterative procedure that re-injects filtered LAT counts residual maps into the model. We confirm that the cosmic-ray proton density varies with the distance from the Galactic center and find a continuous softening of the proton spectrum with this distance. We observe that the Fermi bubbles have a shape similar to a catenary at their bases.