SIMBAD references

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 in the energy range 0.2 TeV<E_γ<2 TeV and in the redshift range 0.05<z<0.4. In these ranges, the form of the absorption coefficient τ(E_γ) 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 Γ_sis converted to one with an observed spectral index Γ_o≃Γ_s+ΔΓ(z), where ΔΓ(z) is a linear function of z in the redshift range 0.05-0.4. We apply this approximation to the spectra of seven TeV blazars.