The following document lists the file abstract/GTAGLIAF_PKS_VA_A.abs
from catalogue VI/111.
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Variability has proven to be an essential aspect of the study of active galactic nuclei and has posed the strongest constraints on theoretical models. Here we propose to study the variability of the prototype BL Lac object PKS 2155-304 at several wavelength bands within the wide range covered by ISO in order to determine variability time-scales and the correlation of variability at different frequencies. A transition from optically thin to optically thick synchrotron emission is expected around about 30 micron. The correlation of light curves above and below this threshold should prove unambiguously whether this is or is not the case. PKS 2155-304 is strongly and rapidly variable at all frequencies above the near IR while it is less strongly variable in the radio band. The proposing team has been involved in multifrequency campaigns, the last of which took place in May 1994. Multifrequency coverage simultaneous to the proposed ISO observations will be attempted. This proposal is complementary to the one submitted by the same team for a detailed spectral study of the source (OBSID: ATREVES, PROPID: PKS_SPEC). The variability study will set constraints on the size of the emitting region with important implications for other physical parameters, like the magnetic field and the beaming factor. This is one of the very few X-ray bright BL Lacs, for which this type of study can be performed due to its high brightness and variability at all energy bands. The ISO range is critical for distinguishing the physical properties of X-ray bright and radio-bright BL Lacs. If the ISO observations should proof that the flat radio to IR spectrum of PKS2155-304 is due to high selfabsorption, a small emission region with high magnetic field would be indicated which may represent a physical distinction with respect to the (expected) thin synchrotron emission in the ISO band from radio bright BL Lacs.