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Long-time-scale X-ray and optical variability is a key characteristic of AGN. Here, we summarise our current understanding of the X-ray and optical continuum variability of radio-quiet AGN and the relation between the two bands. We demonstrate the strong connection between the X-ray variability properties of AGN and the variability of stellar-mass black hole candidates on much shorter time-scales, and discuss the implications of this result for the origins of the variability. The relationship between optical and X-ray variability is complex, with some AGN showing strong X-ray/optical correlations while others show no obvious correlation. We suggest a possible explanation for this variety of behaviour.
Uttley et al. (Thu,) studied this question.