The evolution of Fanaroff-Riley type I (FR I) jets has long been studied in the framework of the FR I-FR II dichotomy. The present paradigm consists of the expansion of overpressured jets in the ambient medium and the generation of standing recollimation shocks, followed by mass entrainment from the external medium that decelerates the jets to subsonic speeds. In this paper, we test the present theoretical and observational models via a relativistic numerical simulation of the jets in the radio galaxy 3C 31. We use the parameters derived from the modelling presented by Laing & Bridle as input parameters for the simulation of the evolution of the source, thus assuming that they have not varied over the lifetime of the source. We simulate about 10 per cent of the total lifetime of the jets in 3C 31. Realistic density and pressure gradients for the atmosphere are used. The simulation includes an equation of state for a two-component relativistic gas that allows a separate treatment of leptonic and baryonic matter. We compare our results with the modelling of the observational data of the source. Our results show that the bow shock evolves self-similarly at a quasi-constant speed, with slight deceleration by the end of the simulation, in agreement with recent X-ray observations that show the presence of bow shocks in FR I sources. The jet expands until it becomes underpressured with respect to the ambient medium, and then recollimates. Subsequent oscillations around pressure equilibrium and generation of standing shocks lead to the mass-loading and disruption of the jet flow. We derive an estimate for the minimum age of the source of t > 1 × 10 8 yr, which may imply continuous activity of 3C 31 since the triggering of its activity. The simulation shows that weak compact steep spectrum sources may be the young counterparts of FR I sources. We conclude that the observed properties of the jets in 3C 31 are basically recovered by the standing shock scenario.

We present the results from an ultra-high-resolution 7 mm Very Long Baseline Array (VLBA) study of the relativistic jet in the BL Lacertae object OJ287 from 1995 to 2011 containing 136 total intensity images. Analysis of the image sequence reveals a sharp jet-position-angle swing by > 100 • during [2004,2006], as viewed in the plane of the sky, that we interpret as the crossing of the jet from one side of the line of sight to the other during a softer and longer term swing of the inner jet. Modulating such long term swing, our images also show for the first time a prominent erratic wobbling behavior of the innermost ∼ 0.4 mas of the jet with fluctuations in position angle of up to ∼ 40 • over time scales ∼ 2 yr. This is accompanied by highly superluminal motions along non-radial trajectories, which reflect the remarkable non-ballistic nature of the jet plasma on these scales. The erratic nature and short time scales of the observed behavior rules out scenarios such as binary black hole systems, accretion disk precession, and interaction with the ambient medium as possible origins of the phenomenon on the scales probed by our observations, although such processes may cause longer-term modulation of the jet direction. We propose that variable asymmetric injection of the jet flow; perhaps related to turbulence in the accretion disk; coupled with hydrodynamic instabilities, leads to the non-ballistic dynamics that cause the observed non-periodic changes in the direction of the inner jet.

Context. The winds from a non-accreting pulsar and a massive star in a binary system collide forming a bow-shaped shock structure. The Coriolis force induced by orbital motion deflects the shocked flows, strongly affecting their dynamics. Aims. We study the evolution of the shocked stellar and pulsar winds on scales in which the orbital motion is important. Potential sites of non-thermal activity are investigated. Methods. Relativistic hydrodynamical simulations in two dimensions, performed with the code PLUTO and using the adaptive mesh refinement technique, are used to model interacting stellar and pulsar winds on scales ∼80 times the distance between the stars. The hydrodynamical results suggest the suitable locations of sites for particle acceleration and non-thermal emission.Results. In addition to the shock formed towards the star, the shocked and unshocked components of the pulsar wind flowing away from the star terminate by means of additional strong shocks produced by the orbital motion. Strong instabilities lead to the development of turbulence and an effective two-wind mixing in both the leading and trailing sides of the interaction structure, which starts to merge with itself after one orbit. The adopted moderate pulsar-wind Lorentz factor already provides a good qualitative description of the phenomena involved in high-mass binaries with pulsars, and can capture important physical effects that would not appear in non-relativistic treatments. Conclusions. Simulations show that shocks, instabilities, and mass-loading yield efficient mass, momentum, and energy exchanges between the pulsar and the stellar winds. This renders a rapid increase in the entropy of the shocked structure, which will likely be disrupted on scales beyond the simulated ones. Several sites of particle acceleration and low-and high-energy emission can be identified. Doppler boosting will have significant and complex effects on radiation.

We present the results from a full polarization study carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) during the first Very Long Baseline Interferometry (VLBI) campaign, which was conducted in 2017 April in the λ3 mm and λ1.3 mm bands, in concert with the Global mm-VLBI Array (GMVA) and the Event Horizon Telescope (EHT), respectively. We determine the polarization and Faraday properties of all VLBI targets, including Sgr A * , M87, and a dozen radio-loud active galactic nuclei (AGNs), in the two bands at several epochs in a time window of 10 days. We detect high linear polarization fractions (2%-15%) and large rotation measures (RM > 10 3.3 -10 5.5 rad m −2 ), confirming the trends of previous AGN studies at millimeter wavelengths. We find that blazars are more strongly polarized than other AGNs in the sample, while exhibiting (on average) order-of-magnitude lower RM values, consistent with the AGN viewing angle unification scheme. For Sgr A * we report a mean RM of (−4.2 ± 0.3) × 10 5 rad m −2 at 1.3 mm, consistent with measurements over the past decade and, for the first time, an RM of (-2.1 ± 0.1) × 10 5 rad m −2 at 3 mm, suggesting