Blazars: Clues to jet physics
  Being dominated by non-thermal emission from aligned relativistic jets, blazars allow us to elucidate the physics of extragalactic jets, and, ultimately, how energy is extracted from the central black hole. Crucial information about jet structure is provided by the spectral energy distributions from radio to gamma-rays, their trends with luminosity, and correlated multifrequency variability. Since blazar jets have broad implications for all radio-loud (and possibly radio-quiet) AGNs, we also need to understand their circumnuclear structure, especially the details of the physical and dynamical conditions of the highly ionized gas on sub-pc scales, which could be directly related to jet formation and radiative power. Eventually, the bulk of information provided by blazars will help us clarify the origin of the radio-loud/radio-quiet AGN dichotomy, one of the most outstanding open issues of extragalactic astrophysics. 
Rita M. Sambruna