The nature of the Galactic diffuse gamma-ray emission as measured by the Fermi Gamma-ray Space Telescope has remained an active area of research for the last several years. In particular, the discovery of a GeV excess towards the Galactic center has generated enormous interest in trying to understand its origins, whether astrophysical or more exotic. While most analyses of the GeV excess confirm its existence, its morphology is not well-constrained, which limits our ability to understand the origin of this excess.
We therefore introduce a new template-fitting approach to study the various components of the Galactic diffuse gamma-ray emission, and their correlations and uncertainties. One application will be to characterize the morphology and the spectrum of the excesses in the inner Galaxy. Rather than starting from fixed predictions from cosmic-ray propagation codes and examining the residuals to understand the quality of fits and the presence of excesses, we introduce additional fine-grained variations in the templates that account for uncertainties in gas tracers and the small scale variations in the density of cosmic rays. This approach results in ~100,000 free parameters for analysis of the Galactic disk, which we fit with an algorithm borrowed from positron emission tomography imaging.
I will present first results from applying this template-fitting approach to the Galactic diffuse gamma-ray emission, including a characterization of the GeV excess in the Galactic center as well as other distinct excesses along the disk.