Aims. The collimation of relativistic jets in galaxies is a poorly understood process. Detailed radio studies of the jet collimation region have been performed so far in a few individual objects, providing important constraints for jet formation models. However, the extent of the collimation zone as well as the nature of the external medium possibly confining the jet are still debated. Methods. In this article, we present a multifrequency and multiscale analysis of the radio galaxy NGC 315, including the use of mm-VLBI data up to 86 GHz, aimed at revealing the evolution of the jet collimation profile. We then consider results from the literature to compare the jet expansion profile in a sample of 27 low-redshift sources, mainly comprising radio galaxies and BL Lacs, which were classified based on the accretion properties as low-excitation (LEG) and high-excitation (HEG) galaxies. Results. We propose that the jet collimation in NGC 315 is completed on sub-parsec scales. A transition from a parabolic to conical jet shape is detected at zt = 0.58 ± 0.28 parsecs or ∼5 × 103 Schwarzschild radii (RS) from the central engine, a distance which is much smaller than the Bondi radius, rB ∼ 92 pc, estimated based on X-ray data. The jet in this and in a few other LEG in our sample may be initially confined by a thick disk extending out to ∼103 − 104RS. A comparison between the mass-scaled jet expansion profiles of all sources indicates that jets in HEG are surrounded by thicker disk-launched sheaths and collimate on larger scales with respect to jets in LEG. These results suggest that disk winds play an important role in the jet collimation mechanism, particularly in high-luminosity sources. The impact of winds on the origin of the FRI and FRII dichotomy in radio galaxies is also discussed.
Context. IceCube has reported a very-high-energy neutrino (IceCube-170922A) in a region containing the blazar TXS 0506+056. Correlated gamma-ray activity has led to the first high-probability association of a high-energy neutrino with an extragalactic source. This blazar has been found to be in a radio outburst during the neutrino event.Aims. Our goal is to probe the sub-milliarcsecond properties of the radio jet right after the neutrino detection and during the further evolution of the radio outburst. Methods. We performed target of opportunity observations at 43 GHz frequency using very long baseline interferometry imaging, corresponding to 7 mm in wavelength, with the Very Long Baseline Array two and eight months after the neutrino event.Results. We produced two images of the radio jet of TXS 0506+056 at 43 GHz with angular resolutions of (0.2 × 1.1) mas and (0.2 × 0.5) mas, respectively. The source shows a compact, high brightness temperature core, albeit not approaching the equipartition limit and a bright and originally very collimated inner jet. Beyond approximately 0.5 mas from the millimeter-VLBI core, the jet loses this tight collimation and expands rapidly. During the months after the neutrino event associated with this source, the overall flux density is rising. This flux density increase happens solely within the core. Notably, the core expands in size with apparent superluminal velocity during these six months so that the brightness temperature drops by a factor of three despite the strong flux density increase. Conclusions. The radio jet of TXS 0506+056 shows strong signs of deceleration and/or a spine-sheath structure within the inner 1 mas, corresponding to about 70 pc to 140 pc in deprojected distance, from the millimeter-VLBI core. This structure is consistent with theoretical models that attribute the neutrino and gamma-ray production in TXS 0506+056 to interactions of electrons and protons in the highly relativistic jet spine with external photons originating from a slower moving jet region. Proton loading due to jet-star interactions in the inner host galaxy is suggested as the possible cause of deceleration.
This is the second of a series of two papers that deepen our understanding of the transversal structure and the properties of recollimation shocks of axisymmetric, relativistic, superfast magnetosonic, overpressured jets. They extend previous work that characterized these properties in connection with the dominant type of energy (internal, kinetic, or magnetic) in the jet to models with helical magnetic fields with larger magnetic pitch angles and force-free magnetic fields. In the first paper of this series, the magnetohydrodynamical models were computed following an approach that allows studying the structure of steady, axisymmetric, relativistic (magnetized) flows using one-dimensional time-dependent simulations. In this paper, synthetic radio images of the magnetohydrodynamical models are produced based on two different models to connect the thermal particle population, modeled by the hydrodynamical code, and the nonthermal particle population (added in post-processing) that causes the synchrotron radiation. The role of the magnetic tension and the Lorentz force in modeling the observational appearance of jets, namely the cross-section emission asymmetries, spine brightening, relative intensity of the knots, and polarized emission is analyzed. A cross-section emission asymmetry caused by a differential change in the angle between the helical magnetic field and the line of sight across the jet width is observed in all models and for both synchrotron emission approximations, as expected from a purely geometrical origin, for viewing angles < 10°. Models with the highest magnetizations and/or magnetic pitch angles lead to an uneven distribution of the internal energy as a consequence of the larger relative magnetic tension and radial Lorentz force, which translates into a spine brightening in the total and linearly polarized intensity maps. Force-free models display a distinct spine brightening that originates in the radial gradient of the axial magnetic field. Highly magnetized jets with large toroidal fields tend to have weaker shocks and correspondingly weaker radio knots. Signatures of this toroidal field can be found in the linearly polarized synchrotron emission for jets with large enough magnetic pitch angles and large enough viewing angles.
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