Anisotropic inverse Compton scattering in the Galaxy

The inverse Compton scattering of interstellar photons off cosmic-ray elect rons seems to play a more important role in the generation of diffuse emiss ion from the Galaxy than thought before. The background radiation field of the Galaxy is highly anisotropic since it is dominated by the radiation fro m the Galactic plane. An observer in the Galactic plane thus sees mostly he ad-on scatterings even if the distribution of the cosmic-ray electrons is i sotropic. This is especially evident when considering inverse Compton scatt ering by electrons in the halo, i.e., the diffuse emission at high Galactic latitudes. We derive formulae for this process and show that the anisotrop y of the interstellar radiation field has a significant effect on the inten sity 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 ext ragalactic background estimates.