Anisotropic inverse Compton scattering in the Galaxy

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