A Simple Analytic Treatment of the Intergalactic Absorption Effect in Blazar Gamma-Ray Spectra

We derive a new and user friendly simple analytic approximation for determining the effect of intergalactic absorption on the γ-ray spectra of TeV blazars the energy range 0.2 TeV < Eγ < 2 TeV and the redshift range 0.05 < z < 0.4. In these ranges, the form of the absorption coefficient τ(Eγ) is approximately logarithmic. The effect of this energy dependence is to steepen intrinsic source spectra such that a source with an approximate power-law spectral index Γs is converted to one with an observed spectral index Γo ≃ Γs +∆Γ(z) where ∆Γ(z) is a linear function of z in the redshift range 0.05 – 0.4. We apply this approximation to the spectra of 7 TeV blazars. Subject headings: Gamma-rays: general – blazars: individual (Mkn 180, PKS 2005-489, PKS 2155-304, H 2356-309, 1ES 1218+30, 1ES 1101-232, PG 1553+113)