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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]

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]