Intergalactic Extinction of High Energy Gamma-Rays
  We discuss the determination of the intergalactic pair-production absorption coefficient (Stecker & De Jager 1998) making use of an empirically based calculation of the IR background spectrum (Malkan & Stecker 1998) which agrees well with recent data and constraints on the IR background. While the Whipple observations of Mrk421 hint at extragalactic absorption, the new HEGRA observations of Mrk501 appear to strongly indicate such absorption. We discuss the determination of absorption at higher redshifts (Salamon & Stecker 1998). We also give a predicted spectrum, with absorption included, for PKS2155-304, which, at a redshift of 0.12, is the highest redshift TeV source yet observed. 
F.W. Stecker (NASA/GSFC)