BL Lacertae contribution to the extragalactic gamma-ray background

Very high energy gamma-rays from blazars traversing cosmological distances through the metagalactic radiation field can convert into electron-positron pairs in photon-photon collisions. The converted gamma- rays initiate electromagnetic cascades driven by inverse-Compton scattering off the microwave background photons. Using a model for the time-dependent metagalactic radiation field consistent with all currently available far-infrared-to-optical data, we calculate the cascade contribution from faint, unresolved high-and low-peaked blazars to the extragalactic gamma- ray background as measured by EGRET. For low-peaked blazars, we adopt a spectral index consistent with the mean spectral index of EGRET detected blazars, and the EGRET luminosity function. For high-peaked blazars, we adopt template spectra matching prototype sources observed with air-Cherenkov telescopes up to 30 TeV, and a luminosity function based on X-ray measurements. The low number of similar to 20 for nearby high-peaked blazars with a flux exceeding 10(-11) cm(-2) s(-1) above 300 GeV inferred from the luminosity function is consistent with the results from air-Cherenkov telescope observations. Including the cascade emission from higher redshifts, the total high-peaked blazar contribution to the observed gamma- ray background at GeV energies can account for similar to 30%.