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The core-shift method is a powerful method to estimate the physical parameters in relativistic jets from active galactic nuclei. The classical approach assumes a conical geometry of jets and a constant plasma speed. However, recent observations have shown that neither may hold close to the central engine, where the plasma in the jet is effectively accelerating and the jet geometry is quasi-parabolic. We modify the classical core-shift method to account for these jet properties. We show that the core-shift index may assume values in the range 0.8−1.2 or 0.53−0.8 depending on the jet geometry and viewing angle, and indices close to both values are indeed observed. We obtain expressions to estimate the jet magnetic field and the total magnetic flux in a jet. We show that the obtained magnetic field value can be easily recalculated down to the gravitational radius scale. For M87 and NGC 315, these values are in good agreement with those obtained by different methods. [ABSTRACT FROM AUTHOR]

The latest results of the most detailed analysis of multi-epoch polarization-sensitive observations of active galactic nuclei (AGN) jets at parsecs scales by very long baseline interferometry (VLBI) reveal several characteristic patterns of linear polarization distribution and its variability [1, 2]. Some of the observed profiles can be reproduced by a simple model of a jet threaded by a helical magnetic field. However, none of the models presented to date can explain the observed polarization profiles with an increase in its degree towards the edges of the jet, and accompanied by a "fountain" type electrical vector pattern and its high temporal variability in the center. Based on simulations of the VLBI observations of relativistic jets, we show here that the observed transverse linear polarization profiles can be naturally produced in a model of jets precessing on ten-years scales. In this scenario, due to the finite resolution of VLBI arrays, the initially strong polarization along the jet axis is blurred due to the superposition of regions whose polarization angle changes significantly when projected onto the sky plane. In our simulations, we qualitatively reproduce the distribution of the electric vector and its variability, though the polarization distribution images are characterized by a bright spine due to weak suppression of polarized emission, which is poorly consistent with observations of quasars. More effective depolarization can be obtained in models with the suppressed emission of the jet spine. [ABSTRACT FROM AUTHOR]

The magnetic field plays a significant role in the phenomenon of highly collimated jets of active galactic nuclei (AGN). Relativistic effects prevent the direct reconstruction of the magnetic field direction as transverse to electric vectors on radio maps. We determined the topology of the B -field by modelling the transverse distributions of the total and linearly polarized intensity, polarization degree, and deviation of the polarization direction from the local jet axis and by further comparison with observational data. We consider (i) a helical field with a different twist angle and (ii) a toroidal field on the jet axis surrounded by a sheath with a longitudinal field. In the latter scenario, we consider different sheath thickness relative to the spine. We assumed the sheath velocity is equal to or less than that of the spine. The relativistic effects have been considered for a general case, under which the axis and velocity vector of the jet and radial directions do not coincide. Our simulations reproduce the main features of the observed transverse profiles of polarization characteristics in parsec-scale AGN jets. The model transverse distribution shapes of the polarization properties are found to be strongly influenced by kinematic and geometric parameters of an outflow. We demonstrated it for three AGNs having different but typical polarization patterns revealed on radio maps. For each of these objects, we identified the model parameters that provide a qualitative correspondence of theoretical profiles with those obtained from observations, indicating that the B -field is strongly ordered on parsec scales. [ABSTRACT FROM AUTHOR]

We carried out a Chandra X‐ray and multi‐frequency very long baseline array (VLBA) study of TXS 0128+554, which is associated with the Fermi γ‐ray source 4FGL J0131.2+5547. The active galactic nuclei (AGN) is unresolved in a 19.3 ks Chandra X‐ray image, and its X‐ray spectrum is well fit by a simple absorbed power law model, with no distinguishable spectral features. Its relatively soft X‐ray spectrum compared to other compact symmetric object (CSOs) may be indicative of a thermal emission component, for which we were able to obtain an upper‐temperature limit of kT <0.08 keV. The compact radio morphology and advanced speed of 0.32c±0.07c indicating a kinematic age of 82 ±17 years, placing TXS 0128+554 among the youngest members of the CSO class. The lack of compact, inverted spectrum hotspots, and an emission gap between the bright inner jet and outer radio lobe structure indicate that the jets have undergone episodic activity, and were re‐launched a decade ago. The predicted γ‐ray emission from the lobes, based on an inverse Compton‐emitting cocoon model, is three orders of magnitude below the observed Fermi‐LAT flux. A comparison to other Fermi‐detected and non‐Fermi detected CSOs with redshift z<0.1 indicates that the γ‐ray emission likely originates in the inner jet/core region, and that nearby, recently launched AGN jets are primary candidates for detection by Fermi. [ABSTRACT FROM AUTHOR]