(on behalf of the Fermi Large Area Telescope Collaboration)
I will describe the development of the model of Galactic diffuse gamma-ray emission for the LAT 4FGL catalog analysis. As for previous LAT catalogs, in particular the 4-year model of Acero et al. (2016), the approach was to fit the Galactic diffuse emission as linear combinations of ‘ring’ templates that trace the distribution of gas across the Milky Way, plus models for the inverse Compton emission and a gamma-ray residual representing non-template components like the Fermi Bubbles and Loop I. The fitting was made with the GaRDiAn tool (Ackermann et al. 2012) and the goal was to improve the systematic uncertainty of the model used for the catalog analysis. A number of refinements were made, some enabled by the availability of improved multiwavelength survey data for defining the templates. The 8-year data set used for the model fitting uses different Pass 8 PSF event types, and different zenith angle limits, for the gamma-ray and exposure data in different energy ranges, to obviate the need to model residual Earth limb emission. Other inputs to the model fitting were also updated, including templates for the lunar and solar emission, as well as the source component of the gamma-ray sky. Some considerations for future refinements of models for Galactic diffuse emission will also be discussed.