SYSTEMATIC EFFECTS IN EXTRACTING A GAMMA-RAY HAZE FROM SPATIAL TEMPLATES

Recent claims of a gamma-ray excess in the diffuse galactic emission detected by the Fermi Large Area Telescope made use of spatial templates from the interstellar medium (ISM) column density and the 408 MHz sky as proxies for neutral pion and inverse Compton (IC) gamma-ray emission, respectively. We identify significant systematic effects in this procedure that can artificially induce an additional diffuse component with a morphology strikingly similar to the claimed gamma-ray haze. To quantitatively illustrate this point, we calculate skymaps of the ratio of the gamma-ray emission from neutral pions to the ISM column density, and of IC to synchrotron emission, using detailed galactic cosmic-ray models and simulations. In the regions above and below the galactic center, the ISM template underestimates the gamma-ray emission due to neutral pion decay by approximately 20%. Additionally, the synchrotron template tends to underestimate the IC emission at low energies (few GeV) and to overestimate it at higher energies (tens of GeV) by potentially large factors that crucially depend on the assumed magnetic field structure of the Galaxy. The size of the systematic effects we find is comparable to the size of the claimed Fermi haze signal. We thus conclude that a detailed model for the galactic diffuse emission is necessary in order to conclusively assess the presence of a gamma-ray excess possibly associated with the Wilkinson Microwave Anisotropy Probe haze morphology.