Detecting the EBL Attenuation of Blazars with GLAST

The Large Area Telescope (LAT) on board GLAST (Gamma‐ray Large Area Space Telescope) due for launch in late 2007 will study the gamma‐ray sky in the energy range 20 MeV to >300 GeV. GLAST‐LAT’s improved sensitivity with respect to previous missions will increase the number of known blazars from about 100 to thousands, with redshifts up to z ∼3–5. Since gamma rays with energy above 10 GeV interact via pair‐production with photons from the Extragalactic Background Light (EBL), the systematic attenuation of GLAST‐detected blazars as a function of redshift would constitute and effective and unique probe to the optical‐UV EBL density and its evolution over cosmic history. Based on the GLAST‐LAT instrument performance, simulations of expected blazar populations attenuated by EBL have been performed.