Attenuation of very high energy gamma rays by the Milky Way interstellar radiation field

The attenuation of very high energy gamma-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 multiwavelength observations by the DIRBE and FIRAS instruments indicates that the energy density of the Galactic interstellar radiation field is higher, particularly in the Galactic center, than previously thought. We have made a calculation of the attenuation of very high energy gamma-rays in the Galaxy, using this new interstellar radiation field, that 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 and is important for both Galactic and extragalactic sources.