We report on high angular resolution polarimetric observations of the nearby radio galaxy M87 using the Very Long Baseline Array at 24 GHz (λ = 1.3 cm) and 43 GHz (λ = 7 mm) in 2017–2018. New images of the linear polarization substructure in the nuclear region are presented, characterized by a two-component pattern of polarized intensity and smooth rotation of the polarization plane around the 43 GHz core. From a comparison with an analogous dataset from 2007, we find that this global polarization pattern remains stable on a time interval of 11 yr, while showing smaller month-scale variability. We discuss the possible Faraday rotation toward the M87 nucleus at centimeter to millimeter wavelengths. These results can be interpreted in a scenario where the observed polarimetric pattern is associated with the magnetic structure in the confining magnetohydrodynamic wind, which also serves as the source of the observed Faraday rotation.

We present a model of blazar variability that can both reproduce smooth large polarization angle swings, and at the same time allow for the seemingly random behavior of synchrotron fluxes, polarization fraction and, occasionally, π/2 polarization jumps. We associate blazar flaring activity with a jet carrying helical magnetic fields and propagating along a variable direction (and possibly with a changing bulk Lorentz factor). The model predicts that for various jet trajectories (i) EVPA can experience large smooth temporal variations while at the same time polarization fraction (Π) can be highly variable; (ii) Π ∼ 0 near sudden EVPA jumps of 90• , but can also remain constant for large, smoother EVPA swings; (iii) the total angle of EVPA rotation can be arbitrary large; (iv) intensity I is usually maximal at points of fastest EVPA changes, but can have a minimum. Thus, even for a regular, deterministic motion of a steadily emitting jet the observed properties can vary in a non-monotonic and/or seemingly stochastic way. Intrinsic fluctuations of the emissivity will further complicated the intensity profiles, but are expected to preserve the polarization structure.

We perform polarimetry analysis of 20 active galactic nuclei (AGN) jets using the Very Long Baseline Array (VLBA) at 1.4, 1.6, 2.2, 2.4, 4.6, 5.0, 8.1, 8.4, and 15.4 GHz. The study allowed us to investigate linearly polarized properties of the jets at parsec-scales: distribution of the Faraday rotation measure (RM) and fractional polarization along the jets, Faraday effects and structure of Faraday-corrected polarization images. Wavelength-dependence of the fractional polarization and polarization angle is consistent with external Faraday rotation, while some sources show internal rotation. The RM changes along the jets, systematically increasing its value towards synchrotron self-absorbed cores at shorter wavelengths. The highest core RM reaches 16,900 rad m −2 in the source rest frame for the quasar 0952+179, suggesting the presence of highly magnetized, dense media in these regions. The typical RM of transparent jet regions has values of an order of a hundred rad m −2 . Significant transverse rotation measure gradients are observed in seven sources. The magnetic field in the Faraday screen has no preferred orientation, and is observed to be random or regular from source to source. Half of the sources show evidence for the helical magnetic fields in their rotating magnetoionic media. At the same time jets themselves contain large-scale, ordered magnetic fields and tend to align its direction with the jet flow. The observed variety of polarized signatures can be explained by a model of spine-sheath jet structure.

We present RadioAstron Space VLBI imaging observations of the BL Lac object S5 0716+71 made on January 3-4 2015 at a frequency of 22 GHz (wavelength λ = 1.3 cm). The observations were made in the framework of the AGN Polarization Key Science Program. The source was detected on projected space-ground baselines up to 70 833 km (5.6 Earth diameters) for both, parallel hand and cross-hand interferometric visibilities. We have used these detections to obtain a full-polarimetric image of the blazar at an unprecedented angular resolution of 24 µas, the highest for this source to date. This enabled us to estimate the size of the radio core to be < 12 × 5 µas and to reveal a complex structure and a significant curvature of the blazar jet in the inner 100 µas, which is an indication that the jet viewing angle lies inside the opening angle of the jet conical outflow. Fairly highly (15%) linearly polarized emission is detected in a jet region of 19 µas in size, located 58 µas downstream from the core. The highest brightness temperature in the source frame is estimated to be > 2.2 × 10 13 K for the blazar core. This implies that the inverse Compton limit must be violated in the rest frame of the source, even for the largest Doppler factor δ ∼ 25 reported for 0716+714.