Context. RadioAstron is a 10 m orbiting radio telescope mounted on the Spektr-R satellite, launched in 2011, performing Space Very Long Baseline Interferometry (SVLBI) observations supported by a global ground array of radio telescopes. With an apogee of ∼350 000 km, it is offering for the first time the possibility to perform µas-resolution imaging in the cm-band. Aims. The RadioAstron Active Galactic Nuclei (AGN) polarization Key Science Project (KSP) aims at exploiting the unprecedented angular resolution provided by RadioAstron to study jet launching/collimation and magnetic-field configuration in AGN jets. The targets of our KSP are some of the most powerful blazars in the sky. Methods. We present observations at 22 GHz of 3C 273, performed in 2014, designed to reach a maximum baseline of approximately nine Earth diameters. Reaching an angular resolution of 0.3 mas, we study a particularly low-activity state of the source, and estimate the nuclear region brightness temperature, comparing with the extreme one detected one year before during the RadioAstron early science period. We also make use of the VLBA-BU-BLAZAR survey data, at 43 GHz, to study the kinematics of the jet in a ∼1.5-year time window. Results. We find that the nuclear brightness temperature is two orders of magnitude lower than the exceptionally high value detected in 2013 with RadioAstron at the same frequency (1.4×10 13 K, source-frame), and even one order of magnitude lower than the equipartition value. The kinematics analysis at 43 GHz shows that a new component was ejected ∼2 months after the 2013 epoch, visible also in our 22 GHz map presented here. Consequently this was located upstream of the core during the brightness temperature peak. Fermi-LAT observations for the period 2010-2014 do not show any γ-ray flare in conjunction with the passage of the new component by the core at 43 GHz. Conclusions. These observations confirm that the previously detected extreme brightness temperature in 3C 273, exceeding the inverse Compton limit, is a short-lived phenomenon caused by a temporary departure from equipartition. Thus, the availability of interferometric baselines capable of providing µas angular resolution does not systematically imply measured brightness temperatures over the known physical limits for astrophysical sources.

Context. A number of extragalactic jets show periodic structures at different scales that can be associated with growing instabilities. The wavelengths of the developing instability modes and their ratios depend on the flow parameters, so the study of those structures can shed light on jet physics at the scales involved. Aims. In this work, we use the fits to the jet ridgeline obtained from different observations of S5 B0836+710 and apply stability analysis of relativistic, sheared flows to derive an estimate of the physical parameters of the jet. Methods. Based on the assumption that the observed structures are generated by growing Kelvin-Helmholtz (KH) instability modes, we have run numerical calculations of stability of a relativistic, sheared jet over a range of different jet parameters. We have spanned several orders of magnitude in jet-to-ambient medium density ratio, and jet internal energy, and checked different values of the Lorentz factor and shear layer width. This represents an independent method to obtain estimates of the physical parameters of a jet. Results. By comparing the fastest growing wavelengths of each relevant mode given by the calculations with the observed wavelengths reported in the literature, we have derived independent estimates of the jet Lorentz factor, specific internal energy, jet-toambient medium density ratio and Mach number. We obtain a jet Lorentz factor γ 12, specific internal energy of ε 10 −2 c 2 , jet-to-ambient medium density ratio of η ≈ 10 −3 , and an internal (classical) jet Mach number of M j ≈ 12. We also find that the wavelength ratios are better recovered by a transversal structure with a width of 10 % of the jet radius. Conclusions. This method represents a powerful tool to derive the jet parameters in all jets showing helical patterns with different wavelengths.

We present Space-VLBI RadioAstron observations at 1.6 GHz and 4.8 GHz of the flat spectrum radio quasar 3C 273, with detections on baselines up to 4.5 and 3.3 Earth Diameters, respectively. Achieving the best angular resolution at 1.6 GHz to date, we have imaged limb-brightening in the jet, not previously detected in this source. In contrast, at 4.8 GHz, we detected emission from a central stream of plasma, with a spatial distribution complementary to the limb-brightened emission, indicating an origin in the spine of the jet. While a stratification across the jet width in the flow density, internal energy, magnetic field, or bulk flow velocity are usually invoked to explain the limb-brightening, the different jet structure detected at the two frequencies probably requires a stratification in the emitting electron energy distribution. Future dedicated numerical simulations will allow the determination of which combination of physical parameters are needed to reproduce the spine-sheath structure observed by Space-VLBI with RadioAstron in 3C 273.

Context. The emission and proper motion of the terminal hotspots of AGN jets can be used as a powerful probe of the intergalactic medium. However, measurements of hotspot-advance speeds in active galaxies are difficult, especially in the young universe, due to the low angular velocities and the low brightness of distant radio galaxies. Aims. Our goal is to study the termination of an AGN jet in the young universe and to deduce physical parameters of the jet and the intergalactic medium. Methods. We use the LOw Frequency ARray (LOFAR) to image the long-wavelength radio emission of the high-redshift blazar S5 0836+710 on arcsecond scales between 120 MHz and 160 MHz.Results. The LOFAR image shows a compact unresolved core and a resolved emission region about 1.5 arcsec to the southwest of the radio core. This structure is in general agreement with previous higher-frequency radio observations with MERLIN and the VLA. The southern component shows a moderately steep spectrum with a spectral index of about −1 while the spectral index of the core is flat to slightly inverted. In addition, we detect for the first time a resolved steep-spectrum halo with a spectral index of about −1 surrounding the core.Conclusions. The arcsecond-scale radio structure of S5 0836+710 can be understood as an FR II-like radio galaxy observed at a small viewing angle. The southern component can be interpreted as the region of the approaching jet's terminal hotspot and the halo-like diffuse component near the core can be interpreted as the counter-hotspot region. From the differential Doppler boosting of both features, we can derive the hotspot advance speed to (0.01 − 0.036) c. At a constant advance speed, the derived age of the source would exceed the total lifetime of such a powerful FR II-like radio galaxy substantially. Thus, the hotspot advance speed must have been higher in the past in agreement with a scenario in which the originally highly relativistic jet has lost collimation due to the growth of instabilities and has transformed into an only mildly relativistic flow. Our data suggest that the density of the intergalactic medium around this distant (z = 2.22) AGN could be substantially higher than the values typically found in less distant FR II radio galaxies.