VARIABILITY OF ACTIVE GALACTIC NUCLEI FROM THE OPTICAL TO X-RAY REGIONS

Some of the progress in understanding the variability of active galactic nuclei (AGNs) from the optical to X-ray regions is reviewed. Although, where there is a clear correlation between variations in the two regions, the optical radiation lags the X-rays, simple reprocessing of the X-ray radiation to produce significant amounts of longer-wavelength continua seems to be ruled out. In a couple of objects where there has been correlated X-ray and optical variability, the amplitude of the optical variability has exceeded the amplitude of the X-ray variability. We suggest that the factor linking the X-ray and optical regions might not be irradiation, but accelerated particles striking matter (as in activity in the solar chromosphere). The diversity in optical-X-ray relationships at different times in the same object, and between different objects, could be explained by evolving differences in local geometry, and by changing directions of motion relative to our line of sight. Linear shot-noise models of the va...