The largest identified class of high-energy gamma-ray sources is a type of Active Galactic Nuclei (AGN) called blazars. The Energetic Gamma Ray Experiment Telescope (EGRET) on board the Compton Gamma Ray Observatory detected more than 50 blazars: GLAST will see thousands. These large redshift extragalactic objects are incredibly powerful sources, each believed to contain a central rotating supermassive black hole with jets of relativistic particles emanating from near its poles. AGN are generally only detected as high-energy gamma-ray sources when one of the jets is directed towards the observer, a geometry for which the accretion torus surrounding the black hole does not obscure us from the black hole or the inner part of the jet. Even for this geometry, however, gamma rays are the only kind of electromagnetic radiation that can directly escape from the central region. Thus, GLAST observations of blazars will provide us with a unique opportunity to study what is happening on the doorsteps of billion-solar-mass black holes.
GLAST studies of high-energy gamma-ray emission from blazars will help answer some of the most important questions we have regarding AGN: