Variability studies of a sample of IntraDay Variable sources

Abstract

The discovery of IntraDay Variability (IDV) in the radio flux from Active Galactic Nuclei gave rise to a series of fundamental issues concerning the origin of the variability. The observation of large flux-density variations on time scales from hours to few days led to the question whether such extreme phenomena can physically be intrinsic to sources. From causality arguments, the variability time scales constrain the maximum size of the emitting region. For IDV sources, this frequently implies very high brightness tempera- tures, far exceeding the inverse-Compton limit. In a source-intrinsic interpretation, only very high Doppler boost factors – of the order of a few tens, or even hundreds – or partic- ular geometries of the emitting regions can explain such short time scale variability. No strong evidence in favour of the second hypothesis has been found yet, while the first one seems to contradict the results of VLBI observations. Source-extrinsic models, instead, ascribe the variability to propagation effects. Recent studies seem to indicate that inter- stellar scintillation plays an important role in the IDV shown by several sources, but it is insufficient to explain all the aspects of the phenomenon.<br /> For this thesis project, a sample of IDV sources has been monitored for more than three years with the Urumqi radio telescope, at a frequency of 4.8GHz, aiming to study the evolution of their variability characteristics and to compare it with the predictions of source-intrinsic and source-extrinsic IDV models. 22 observing sessions have been per- formed between August 2005 and April 2008 for a total of 78 days of observations. All the variability curves have been analyzed by means of classical and new meth- ods of time-series analysis. The results seem to point towards an extrinsic origin of the variability for three out of the four main targets of the monitoring campaign. For the blazar AO 025+164, instead, there is strong evidence that the evolution of its variability characteristics is correlated with changes in the structure of the source.<br /> A search for possible regular flux-density variations in IDV sources led to the discovery of a 1-day periodic contribution to their variability, which can not be ascribed to any known systematic effect. Plausibly, it can be related to variations in the total electron content of the ionosphere, or it may be caused by the magnetosphere. However, the issue is still open.