Anisotropic Inverse Compton Scattering in the Galaxy

The inverse Compton scattering of interstellar photons off cosmic-rayelectrons seems to play a more important role in the generation of diffuseemission from the Galaxy than thought before. The background radiation field ofthe Galaxy is highly anisotropic since it is dominated by the radiation fromthe Galactic plane. An observer in the Galactic plane thus sees mostly head-onscatterings even if the distribution of the cosmic-ray electrons is isotropic.This is especially evident when considering inverse Compton scattering byelectrons in the halo, i.e. the diffuse emission at high Galactic latitudes. We derive formulas for this process and show that the anisotropy of theinterstellar radiation field has a significant effect on the intensity andangular distribution of the Galactic diffuse gamma rays, which can increase thehigh-latitude Galactic gamma-ray flux up to 40%. This effect should be takeninto account when calculating the Galactic emission for extragalacticbackground estimates.