We study the geometric and topological properties of strange non-chaotic attractors created in non-smooth saddle-node bifurcations of quasiperiodically forced interval maps. By interpreting the attractors as limit objects of the iterates of a continuous curve and controlling the geometry of the latter, we determine their Hausdorff and box-counting dimension and show that these take distinct values. Moreover, the same approach allows us to describe the topological structure of the attractors and to prove their minimality.

We report about first results of the RoboPol project. RoboPol is a large-sample, high-cadence, polarimetric monitoring program of blazars in optical wavelengths, using a camera specifically constructed for this project, mounted at the University of Crete's Skinakas Observatory 1.3 m telescope. The analysis of RoboPol data is conducted in conjunction with Fermi LAT gamma-ray data, and multifrequency radio data from the OVRO (Caltech), F-GAMMA (MPIfR), and Torun (NCU) monitoring programs. Using carefully selected samples of gamma-ray bright and weak blazars we investigate a connection between their optical polarization behaviour and variability properties in gamma. We examine a relationship of gamma flares with polarization angle rotations relying on robust statistical criteria. We analyse also the optical polarization variability itself in order to establish some restrictions on physical models of blazars jets.

There is new evidence which identifies seasonal changes of the variability time scale in intraday variable (IDV) sources with refractive interstellar scintillation effects. Such a RISS model takes the annual change of the Earth's velocity relative to the scattering medium into account. In September 1998 we found a remarkable prolongation of the variability time scale in the IDV source 0917+624 with only small variations in flux density during a period of 5 days. This was explained as a seasonal effect, in which the velocity vector of the Earth and the interstellar medium nearly cancelled. In order to further investigate the applicability of the model for 0917+624, we performed an Effelsberg 6 cm-flux monitoring program over the course of one year. Since September 2000, the source appears to be remarkably inactive and yet (May 2001) no return to its normal, faster and stronger variability pattern is observed. Here, our observational results and a possible explanation for the current quiescence are presented.

We summarise results from flux density monitoring campaigns performed with the 100 m radio telescope at Effelsberg and the VLA during the past 15 yrs. We briefly discuss some of the statistical properties from now more than 40 high declination sources (δ ≥ 30°), which show intraday variability (IDV). In general, IDV is more pronounced for sources with flat radio spectra and compact VLBI structures. For 0917+62, we present new VLBI images which suggest that the variability pattern is modified by the occurrence of new jet components. For 0716+71, we show the first detection of IDV at millimetre wavelengths (32 GHz). For the physical interpretation of the IDV phenomenon, a complex source and frequency dependent superposition of interstellar scintillation and source intrinsic variability should be considered.

Radio variability on timescales from a few hours to several days in extragalactic flat spectrum radio sources is generally classified as intraday variability (IDV). The origin of this short term variability is still controversial and both extrinsic and intrinsic mechanisms must be considered and may both contribute to the observed variations. The measured linear and circular polarisation of IDV sources constrains the low energy end of the electron population. Any population of cold electrons within sources at or above the equipartition temperature of 1011 K depolarises the emission and can be ruled out. Intrinsic shock models are shown to either violate the large fraction of sources displaying IDV or they do not relax the light travel time argument for intrinsic variations. From structure function analysis, we further conclude that interstellar scintillation also leads to tight size estimates unless a very local cloud in the ISM is responsible for IDV.

Narrow-Line Seyfert 1 (NLS1) class of active galactic nuclei (AGNs) is generally radio-quiet, but a small percent of them are radio-loud. The recent discovery by Fermi/LAT of high-energy γ-ray emission from 4 NLS1s proved the existence of relativistic jets in these systems. It is therefore important to study this new class of γ-ray emitting AGNs. Here we report preliminary results about the observations of the July 2010 γ-ray outburst of PMN J0948+0022, when the source flux exceeded for the first time 10−6 ph cm−2 s−1 (E > 100 MeV).

We present the science case for ICE-T, the International Concordia Explorer Telescope, a double telescope optimized for ultra-high and ultra wide field optical time-series photometry fromDome C. ICE-T consists of two 60 cm 8°×8°-FOV Schmidt telescopes for Sloan g and i photometry and one independent 25 cm, solar, full-disk, Ca ii-K imagingtelescope on a single mount. ICE-T is therefore operable during night and day. A 28 cm narrow-field Maksutov spectrophotometric telescope for night-time aerosol measurements (TAVERN-SP) will beprovided by AWI early on and operated in parallel with IRAIT and later with ICE-T. The low scintillation noise and the long continuous darkness are among the unique properties for highprecision optical time-series photometry.