X-ray variability of active galactic nuclei.

Data on the X-ray variability of active galactic nuclei (AGNs) are reviewed, and new data on Einstein observations of BL Lac variability are presented. All the results are discussed in terms of a statistical quantification of the observations. It is concluded that about one-third of AGNs show X-ray variability. The characteristic variability time scale for AGNs is inferred to lie between 1000 and ten million seconds. The largest variations and the largest percentage of variable objects occur among BL Lac/OVV objects, as opposed to Seyferts and quasars.