X-ray variability of AGN and its relationship to the Optical/UV

Some models predict a connection between the X-ray emission of active galactic nuclei (AGN) and lower-energy radiation in the optical and ultraviolet (OUV), which can be tested by multiwaveband observations. Strong correlations are observed within the optical/UV band and between the extreme ultraviolet (EUV) and X-rays. The relationship between the X-ray and OUV fluxes is much less clear, with some recent intensive campaigns showing little or no correlation. Furthermore, the X-rays usually show very rapid variations which are not seen in the OUV. We show, however: that before drawing conclusions about possible emission mechanisms, it is important to consider the X-ray spectrum and its variability in detail. The current data strongly favor thermal Comptonization as the origin of the X-rays, with the OUV acting: as the seed photons. Reprocessing: is not ruled out as a mechanism for producing relatively rapid variations in OUV flux, and these can feed back into the Compton upscattering process. The most rapid variability in the X-ray band is almost certainly unrelated to the OUV seed photons, however, and probably originates intrinsically in the the X-ray corona.