We derive a new and user friendly simple analytic approximation for determining the effect of intergalactic absorption on the \gamma -ray spectra of TeV blazars the energy range 0.2 ~ { } { TeV } < E _ { \gamma } < 2 TeV and the redshift range 0.05 < z < 0.4 .
In these ranges , the form of the absorption coefficient \tau ( E _ { \gamma } ) 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 \Gamma _ { s } is converted to one with an observed spectral index \Gamma _ { o } \simeq \Gamma _ { s } + \Delta \Gamma ( z ) where \Delta \Gamma ( 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 .