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Paper: BL Lac Objects and Blazars: Past, Present and Future
Volume: 159, BL Lac Phenomenon
Page: 3
Authors: Urry, M.
Abstract: The past 20 years have seen phenomenal progress in our understanding of BL Lac objects. They form part of the blazar class, which are radio-loud AGN distinguished by relativistic jets aligned along our line of sight. Several critical milestones have helped establish this picture, first proposed at the Pittsburgh meeting 20 years ago, particularly the EGRET and TeV detections of beamed gamma-ray emission. The spectral energy distributions are double peaked and follow a self-similar sequence in luminosity, which can be explained by electron cooling on ambient photons. This simple paradigm has yet to be tested, and further questions remain, notably about physical conditions in blazar jets --- the kinetic power, magnetic energy density, acceleration time scales, proton content, etc. --- and how this energy is transported in the innermost regions. We also do not know what kinds of jets nature makes, i.e., the relative number densities of low- (``blue'' BL Lacs) or high-luminosity (``red'' BL Lacs and FSRQ) blazars. Since blazars are representative of all radio-loud AGN, their jet properties have broad implications. Thus we would also like to understand the circumnuclear structure, especially the details of hot, highly ionized, high velocity gas on sub-parsec scales, which could play a role in jet dynamics and could possibly affect the formation of FRI vs. FRII type jets. The study of blazars should eventually help us understand the difference between radio-quiet and radio-loud AGN.
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