Attenuation of Vhe Gamma Rays by the Milky Way Interstellar Radiation Field

The attenuation of very high energy γ-rays by pair production on the Galactic interstellar radiation field has long been thought of as negligible. However, a new calculation of the interstellar radiation field consistent with multi-wavelength observations by DIRBE and FIRAS indicates that the Galactic interstellar radiation field is intense. We have made a calculation of the attenuation of very high energy γ-rays in the Galaxy using this new interstellar radiation field which takes into account its nonuniform spatial and angular distributions. We find that the maximum attenuation occurs around 100 TeV at the level of about 25% for sources located at the Galactic center, within the energy range of the HESS instrument, and is important for both Galactic and extragalactic sources. Subject headings: Galaxy: general — gamma-rays: observations — gamma-rays: theory — radiation mechanisms: general — radiative transfer