One of the general characteristics of Active Galactic Nuclei (AGN) such as quasars, blazars, and Seyfert galaxies is that their brightness tends to vary. In fact, it is generally assumed that most, if not all, AGN are variable at some level. For example, virtually all of the AGN 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, (also referred to as blazars) are among the most 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 AGN.
Variability studies of AGN typically focus on one of three time scales. These time scales are related to the nature of the data and to the logistics and practical matters associated with obtaining astronomical observations; they are not necessarily related in any meaningful way to the physical processes producing the variability. When discussing AGN variability we typically use long term, interday, and microvariability to differentiate the three regimes.
Time Scale Typical time scale for viewing the data Typical time resolution Long Term Several years or decades Months or years Interday Several days or weeks Days Microvariability Several hours Minutes