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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]

We analysed the parsec-scale linear polarization properties of 436 active galactic nuclei (AGNs) based on 15 GHz polarimetric Very Long Baseline Array observations. We present polarization and total intensity images averaged over at least five epochs since 1996 January 19 through 2019 August 4. Stacking improves the image sensitivity down to ∼30 μJy beam−1 and effectively fills out the jet cross-section both in total intensity and linear polarization. It delineates the long-term persistent magnetic field configuration and its regularity by restoring spatial distributions of the electric vector position angle (EVPA) and fractional polarization, respectively. On average, about 10 yr of stacking period is needed to reveal the stable and most-complete polarization distribution of a source. We find that the degree of polarization significantly increases down and across the jet towards its edges, typically manifesting U or W-shaped transverse profiles, suggesting a presence of a large-scale helical magnetic field associated with the outflow. In some AGN jets, mainly BL Lacs, we detect quasi-constant fractional polarization profiles across the jet, accompanied by EVPAs that closely follow the outflow. BL Lacs show higher fractional polarization values in their cores and jets than those in quasars up to hectoparsec de-projected scales, while on larger scales, they become comparable. High-synchrotron-peaked BL Lac jets are found to be less polarized than intermediate and low-synchrotron-peaked BL Lacs. The spatial distribution of the EVPAs in BL Lacs tend to align with the local jet direction, while quasars show an excess of orthogonal polarization orientation. [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]

Within the MOJAVE VLBA programme (Monitoring of Jets in AGN with VLBA Experiments), we have accumulated observational data at 15 GHz for hundreds of jets in gamma-ray bright active galactic nuclei since the beginning of the Fermi scientific observations in 2008 August. We investigated a time delay between the flux density of AGN parsec-scale radio emission at 15 GHz and 0.1–300 GeV Fermi LAT photon flux, taken from constructed light curves using weekly and adaptive binning. The correlation analysis shows that radio is lagging gamma-ray radiation by up to 8 months in the observer's frame, while in the source frame, the typical delay is about 2–3 months. If the jet radio emission, excluding the opaque core, is considered, significant correlation is found at greater time lags. We supplement these results with VLBI kinematics and core shift data to conclude that the dominant high-energy production zone is typically located at a distance of several parsecs from the central nucleus. We also found that quasars have on average more significant correlation peak, more distant gamma-ray emission region from the central engine and shorter variability time-scale compared to those of BL Lacertae objects. [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]

Using stacking of images obtained at different epochs, we studied the variability properties of linear polarization of active galactic nucleus (AGN) jets on parsec-scales. Our sample is drawn from the MOJAVE programme, and consists of 436 AGNs manifesting core-jet morphology and having at least five VLBA observing epochs at 15 GHz from 1996 January through 2019 August, with some additional archival VLBA data reduced by us. We employed a stacking procedure and constructed maps of (i) standard deviation of fractional polarization and electric vector position angle (EVPA) over epochs as the measure of variability and (ii) median polarization degree to quantify typical values in time. The distributions of these values along and across the jet were analysed for the whole sample for the first time. We found that core EVPA variability is typically higher than that of the jet, presumably due to component blending and outflow bends in the core. The BL Lacertae object cores have lower EVPA variability, compared to that of quasars, possibly due to lower Faraday rotation measure, suggesting a stronger ordered magnetic field component. The EVPA becomes more stable down the jet. Most of the sources showing this trend have a time coverage of more than 12 yr and at least 15 epochs. The possible cause could be the increase of stability in the magnetic field direction, reflecting an increase in the fraction of the magnetic field that is ordered. There are no significant optical-class-dependent or spectral-class-dependent relations in the EVPA variability properties in AGN jets. [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]