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One of the general characteristics of Active Galactic Nuclei (AGNs) such as
quasars, blazars, and Seyfert
galaxies is that they seem to be variable in brightness
It is often assumed that most, if not all, AGNs are variable
at some level. For example, virtually all of the AGNs observed
in the Hubble Deep Field were determined to exhibit detectable
variability over a period of two years. Furthermore, some categories
of AGN known as BL Lacs, (refered to as
blazars) are among the most extremely active variable sources
known in the universe. Based on the limitations of the existing
data and analysis, some systematic observational material would
substantially improve our understanding of the nature of the
variability of AGNs.
Variability Time Scales
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