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Abstract We present an analysis of the cross-correlation between optical brightness and polarization degree in different types of blazars. The aim is to identify objects with simultaneous and consistent changes in characteristics and to determine if this behavior relates to the types of objects studied. The analysis includes 23 objects: 11 Flat Spectrum Radio Quasars (FSRQs), 11 BL Lacs, and 1 radio galaxy. Dense overlapping observation series in theRband were used, collected over more than 10 yr as part of a monitoring program for bright blazars at St. Petersburg State University. The cross-correlation analysis procedure is detailed, including a method for assessing significance based on Monte Carlo simulations of synthetic light curves modeled using a Damped Random Walk. Significant correlations were found for 5 FSRQs and 1 BL Lac. No significant correlation was detected for 10 BL Lacs and 6 FSRQs. One object did not yield a reliable estimate. Based on the current results, we cannot claim that the observed difference in the behavior of these emission characteristics for different classes of blazars is significant. It is possible that observed correlations may be explained by the contribution of simultaneous flare events to the changes in flux and polarization degree curves, which occur more frequently in FSRQs. 

Blazars are a subclass of active galactic nuclei (AGNs) with a high optical linear polarization that originates in relativistic jets. Polarization parameters such as the degree of polarization (PD) and the electric vector position angle (EVPA) are directly related to the properties of the magnetic field in the jets. A study of the optical polarization of blazars allows conclusions to be drawn about the field geometry, its evolution, and its relation to the emission properties of the blazars. The periods of ordered changes in the electric vector position angle, so-called rotations, are of particular interest. We used a new method to determine EVPA rotations and to estimate their statistical significance with the aim to analyze long-term polarimetric observations of five blazars: OJ 287, S5 0716+71, 3C 454.3, CTA 102, and PG 1553+113. This resultes in the identification of 256 EVPA rotations. We found possible tendencies for the EVPA rotations to occur in a preferred direction in each of these sources: clockwise for OJ 287 and CTA 102, and counterclockwise for the others. The EVPA rotations can be explained by the spiral structure of the magnetic field in the jet. In this case, the observed preferred direction of rotations reflects the global structure of the magnetic field, which can be associated with the direction of rotation of either the black hole ergosphere or the accretion disk. 

Context.Blazars are beamed active galactic nuclei (AGNs) known for their strong multi-wavelength variability on timescales ranging from years down to minutes. Many different models have been proposed to explain this variability. Aims.We aim to investigate the suitability of the twisting jet model presented in previous works to explain the multi-wavelength behaviour of BL Lacertae, the prototype of one of the blazar classes. According to this model, the jet is inhomogeneous, curved, and twisting, and the long-term variability is due to changes in the Doppler factor due to variations in the orientation of the jet-emitting regions. Methods.We analysed optical data of the source obtained during monitoring campaigns organised by the Whole Earth Blazar Telescope (WEBT) in 2019–2022, together with radio data from the WEBT and other teams, andγ-ray data from theFermisatellite. In this period, BL Lacertae underwent an extraordinary activity phase, reaching its historical optical andγ-ray brightness maxima. Results.The application of the twisting jet model to the source light curves allows us to infer the wiggling motion of the optical, radio, andγ-ray jet-emitting regions. The optical-radio correlation shows that the changes in the radio viewing angle follow those in the optical viewing angle by about 120 days, and it suggests that the jet is composed of plasma filaments, which is in agreement with some radio high-resolution observations of other sources. Theγ-ray emitting region is found to be co-spatial with the optical one, and the analysis of theγ-optical correlation is consistent with both the geometric interpretation and a synchrotron self-Compton (SSC) origin of the high-energy photons. Conclusions.We propose a geometric scenario where the jet is made up of a pair of emitting plasma filaments in a sort of double-helix curved rotating structure, whose wiggling motion produces changes in the Doppler beaming and can thus explain the observed multi-wavelength long-term variability.