Variability in the Continuum of Active Galactic Nuclei

Abstract

Variability in the continuum of Active Galactive Nuclei (AGN) is briefly reviewed. Emphasis is put on the importance of systematic, broad band, coordinated monitoring observations to the investigation of the basic structural properties and radiation mechanisms of the continuum source as well as the origins of the observed variations.Two particular problems are discussed in some detail. The first concerns the case for relativistic beaming in BL Lac type objects and the critical observations in the radio and X-ray bands needed to establish the case are pointed out. The second concerns the constraints which follow from variability studies on non-thermal models of the optical-ultraviolet and of the X-ray emission in Seyfert galaxies.