Long-term X-ray variability in blazars and its multi- waveband context

The accumulation of well-sampled X-ray, optical, and radio light curves of blazars and coredominated radio galaxies allows analysis of the long-term variability properties and crossfrequency correlations. Armed with this rich database, we are starting to be able to determine where in the jet the nonthermal flares take place. We find that these can occur in at least two locations: in the core seen on millimeter-wave VLBI images and in a region with a helical magnetic field well upstream of the core. The latter is consistent with the jet acceleration and collimation zone according to the magnetohydrodynamical models favored by most theorists. In this picture, the jet of a blazar, which points within several degrees of the line of sight, produces the bulk of its radiation near the end of this zone and in standing shocks or other compressions downstream. The outbursts and superluminal radio knots are connected with events in the central engine signalled by dips in the X-ray flux. However, a model detailing how the accretion-disk/black-hole system couples with the jet still needs to be developed.