Absorption of very high energy gamma rays in the Milky Way

Abstract

Galactic gamma ray astronomy at very high energy (E<SUB>γ</SUB>≳30 TeV ) is a vital tool in the study of the nonthermal universe. The interpretation of the observations in this energy region requires the precise modeling of the attenuation of photons due to pair production interactions (γ γ →e<SUP>+</SUP>e<SUP>-</SUP> ) where the targets are the radiation fields present in interstellar space. For gamma rays with energy E<SUB>γ</SUB>≳300 TeV the attenuation is mostly due to the photons of the cosmic microwave background radiation. At lower energy the most important targets are infrared photons with wavelengths in the range λ ≃50 - 500 μ m emitted by dust. The evaluation of the attenuation requires a good knowledge of the density, and energy and angular distributions of the target photons for all positions in the Galaxy. In this work we discuss a simple model for the infrared radiation that depends on only few parameters associated to the space and temperature distributions of the emitting dust. The model allows to compute with good accuracy the effects of absorption for any space and energy distribution of the diffuse Galactic gamma ray emission. The absorption probability due to the Galactic infrared radiation is maximum for E<SUB>γ</SUB>≃150 TeV , and can be as large as P<SUB>abs</SUB>≃0.45 for distant sources on lines of sight that pass close to the Galactic center. The systematic uncertainties on the absorption probability are estimated as ∆ P<SUB>abs</SUB>≲0.08 .