Variability is an important tool for the diagnostics of physical processes
in AGN. The extended lifetime of ISO provides us with a unique opportunity
to study the variability of well-defined subsets of sources in order to
distinguish between competing non-thermal and thermal emission mechanisms
in the infrared. We propose to obtain second epoch photometric data with
ISOPHOT for 54 AGNs already observed in cycle 1. We will
then compare the variability characteristics of the sub-samples. Thermal
emission from dust cannot vary on short time scales, as the emission medium
is very extended. On the other hand, non-thermal emission mechanisms like
synchrotron emission are able to vary significantly on very short time
scales. With two observations (including cycle 1) we expect to find
statistical evidence for variability in objects dominated
by non-thermal processes.
The selected sample contains 27 radio-quiet objects and 27 radio-loud
objects. Radio-quiet sources are thought to be dominated by dust emission.
Variations on the time-scale of a year are therefore not expected. However
synchrotron emission is the usual explanation for the far-infrared and
millimetre emission of radio-loud sources, which should therefore exhibit
variability. This simple prediction is central to the models and very
important to test. This proposal aims at testing this paradigm in a very
efficient way.
We build a sample from the objects observed in the first cycle by Wilkes et
al., Courvoisier et al., and Chini et al. Pooling together these samples
allows us to obtain statistically meaningful samples in both radio-quiet
and radio-loud categories. We propose to observe the objects in 3 broad-band
filters with ISOPHOT from 10 to 200 microns depending on the object to test
the variability of the AGN as well as detect any spectral dependence.