Unified Schemes and the two Classes of BL Lacs

Abstract

I briefly summarize the main tenets of unified schemes of BL Lacs and low-luminosity radio galaxies, discussing in particular the evolution of this field after the Como 1988 meeting. I also examine some of the open problems and complications of the simplest scheme. Finally, the question of the existence of two classes of BL Lacs and our related change of perspective in the past few years are also addressed.

12 pages, 1 figure. Invited talk at the Turku Conference on the BL Lac Phenomenon (June 1998), ed. L. Takalo, ASP Conference Series, in press. Postscript file also available at http://icarus.stsci.edu/~padovani/bllacs_papers.html