BL Lac Objects and Blazars: Past, Present, and Future

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 whose relativistic jets are aligned along our line of sight. Several critical milestones have helped establish this picture, first proposed at the Pittsburgh BL Lac meeting 20 years ago, most recently 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. Some clues are available from multiwavelength monitoring campaigns although better sampling over longer periods is clearly called for. Recent work on the host galaxies of BL Lac objects supports their unification with low-power, aligned FRI radio galaxies. Nature makes jets with a large range of kinetic powers, but the relative number densities of low-luminosity ("blue" BL Lacs) or high-luminosity ("red" BL Lacs and FSRQ) blazars --- is highly uncertain. Since according to unified schemes blazars are representative of all radio-loud AGN, their jet properties have broad implications. Future EUV/X-ray observations will illuminate the circumnuclear structure, especially the 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 may also help us eventually understand the difference between radio-quiet and radio-loud AGN. [slightly abridged]