Temporal characteristics of Blazar variability

Wagner, S.

Landessternwarte, K\"onigstuhl, Heidelberg, Germany


Variability is one of the defining characteristics of Blazars. Changes
in the observed intensity, spectral slopes, and polarization characteristics
have been observed throughout the entire electromagnetic spectrum and on
all time-scales probed for variability. The power density spectra are 
very broad, indicating that either a variety of different processes 
contribute to the observed variations or that the parameters controlling
variability time scales vary rather homogeneously over a wide range.
While strict periodicity has not been detected so far, a large fraction
of Blazars exhibits distinct features in their power spectra, indicative 
of a preferred time-scale. These features permit a distinction between
the possible explanations for the broad power spectra and constrain
details of either of those.
In this talk the statistical characteristics of Blazar variability will
be reviewed. Particular emphasis will be placed on the X-ray regime. 
Important progress has been made in recent years, and perspectives for
future programs will be discussed.