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Variability of active galactic nuclei from the optical to X-ray regions

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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 variability are ruled out; instead there must be large-scale organization of variability. Variability occurs on light-crossing timescales rather than viscous timescales and this probably rules out the standard Shakura-Sunyaev accretion disc. Instead, we believe that the main energy generation mechanism is probably electromagnetic. The overall average continuum shape appears to be the same in both radio-loud and radio-quiet AGNs, strongly suggesting a similar origin to the continua. Radio-loud and radio-quiet AGNs have quite similar optical variability properties, and this suggests a common variability mechanism. Beaming effects could be significant in all types of AGN. Despite their extreme X-ray variability properties, our observations show that narrow-line Seyfert 1 (NLS1) galaxies do not show extreme optical variability, and that their optical variability properties could well be similar to those of non-NLS1 galaxies.
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