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We conducted multi-epoch, multi-frequency parsec-scale studies on the gigahertz-peaked spectrum quasar PKS 0858-279 with the Very Long Baseline Array (VLBA). Our observations on 2005-11-26 elucidated a weak core, characterized by an inverted spectrum, and a distinctly bent jet that exhibited a notable bright feature in its Stokes I emission. Through comprehensive analysis of polarization and spectral data, we inferred the formation of a shock wave within this feature, stemming from interactions with a dense cloud in the ambient medium. In this paper, VLBI-Gaia astrometry further reinforces the core identification. With a deep analysis of six additional VLBA epochs spanning from 2007 to 2018, we observed that while the quasar's parsec-scale structure remained largely consistent, there were discernible flux density changes. These variations strongly imply the recurrent ejection of plasma into the jet. Complementing our VLBA data, RATAN-600 observations of the integrated spectra suggested an interaction between standing and travelling shock waves in 2005. Moreover, our multi-epoch polarization analysis revealed a drastic drop in rotation measure values from 6000 rad/m^2 to 1000 rad/m^2 within a single year, attributable to diminishing magnetic fields and particle density in an external cloud. This change is likely instigated by a shock in the cloud, triggered by the cloud's interaction with the jet, subsequently prompting its expansion. Notably, we also observed a significant change in the magnetic field direction of the jet, from being perpendicular post its observed bend to being perpendicular prior to the bend - an alteration possibly induced by the dynamics of shock waves., Comment: 14 pages, 10 figures, accepted for publication in MNRAS

Aims. Relativistic jets launched from active galactic nuclei accelerate up to highly relativistic velocities within a few parsecs to tens of parsecs. The precise way in which this process takes place is still under study. While magnetic acceleration is known to be able to accelerate relativistic outflows, little attention has been paid to the role of thermal acceleration. The latter has been assumed to act only on compact regions, very close to the central engine, and to become negligible on parsec scales. However, this holds under the assumption of small internal energies as compared to the magnetic ones, and whether this is true or what happens when we drop this assumption is currently uncertain. Methods. We use a 2D relativistic magnetohydrodynamical code to explore jet acceleration from sub-parsec to parsec scales. As initial conditions for our models, we use observational constraints on jet properties derived by means of very long baseline interferometry observations for a Fanaroff Riley I radio galaxy, NGC\,315. We investigate the parameter space established for this source and perform a number of simulations of magnetically, thermally or kinetically dominated jets at injection, and compare our results with the observed ones. Results. Our simulated jets show that when thermal energy is comparable to or exceeds magnetic energy, thermal acceleration becomes significant at parsec scales. This result has important consequences, potentially extending the acceleration region far beyond the collimation scales, as thermal acceleration can effectively operate within a conically expanding jet. In all the models, we observe acceleration to be driven by expansion, as expected. A number of our models allow us to reproduce the acceleration and opening angles observed in NGC\,315. Finally, our results indicate that disk-launched winds might play an important role in the jet propagation., Comment: 25 pages, 16 figures. Accepted for publication in A&A

Aims. Hot accretion flows are thought to be able to power the relativistic jets observed in Active Galactic Nuclei. They can present themselves as SANE (Standard And Normal Evolution) disks or MAD (Magnetically Arrested Disks), two states implying profound differences in the physical properties of the disks themselves and of the outflows they produce. Methods. In this paper we use a multi-frequency and multi-epoch data set to study the giant radio galaxy NGC 315, with the goal to explore the properties of its accretion disk and sub-parsec jet. We analyze the source maps with a pixel-based analysis and we use theoretical models to link the observational properties of the jet to the physical state of the accretion disk. Results. We propose that the bulk flow in NGC 315 accelerates on sub-pc scales, concurrently with the parabolic expansion. We show that this fast acceleration can be theoretically reconciled with a magnetically driven acceleration. Along the acceleration and collimation zone, we observe an unexpected spectral behavior, with very steep spectral index values $\alpha \sim -1.5$ ($S_\nu \propto \nu^\alpha$) between 22 GHz and 43 GHz. Based on the properties of this region, we predict the black hole of NGC 315 to be fast rotating and the magnetic flux threading the accretion disk to be in excellent agreement with that expected in the case of a MAD. Using a new formalism based on the core-shift effect, we model the magnetic field downstream a quasi-parabolic accelerating jet and we reconstruct it up to the event horizon radius. In the MAD scenario, we compare it with the expected magnetic saturation strengths in the disk, finding a good agreement., Comment: 12 pages plus appendix, 8 figures. Accepted for publication in A&A

Studies of the most distant AGNs allow us to test our current understanding of the physics present in radio-jetted AGNs across a range of environments. The decrease in apparent luminosity with distance is the primary difficulty to overcome in the study of these distant AGNs, which requires highly sensitive instruments. Our goal is to employ new long wavelength radio data to better parametrise the broad-band SED of GB 1508+5714, a high-redshift (z=4.30) AGN. Its high redshift, high intrinsic luminosity, and classification as a blazar allow us to test emission models that consider the efficient cooling of jet electrons via inverse Compton losses in interactions with the dense CMB photon field at high redshifts. A significant detection of this effect in GB 1508+5714 may partly explain the apparent sparsity of high-redshift radio galaxies in wide-field surveys; detections of this kind are only becoming possible with the current generation of SKA precursors. We used international LOFAR telescope to image the long wavelength radio emission around the high-redshift blazar GB 1508+5714 on arcsecond scales at frequencies between 128 MHz and 160 MHz. This allowed us to compare the spatially resolved structure with higher frequency observations, and to construct spectral index maps. The LOFAR image shows a compact unresolved core and two resolved emission regions around 2 arcsec to the east and to the west of the radio core. We find structure consistent with previous VLA observations, as well as a previously unreported emission region to the east. We interpret the arcsecond-scale radio structure of GB 1508+5714 as a FR II-like radio galaxy at a small viewing angle. Our SED modelling shows that a scenario featuring significant quenching effects caused by interaction with the CMB provides a good description of the data, and notably explains the suppressed radio emission., Comment: 11 pages, 10 figures, 2 tables

The interaction of jets in High-Mass X-ray Binaries (HMXBs) with the strong winds driven by the hot companion star in the vicinity of the compact object is fundamental to understand the jet dynamics, non-thermal emission and long-term stability. However, the role of the jet magnetic field in this process is unclear. We study the dynamical role of weak and moderate-to-strong toroidal magnetic fields during the first hundreds of seconds of jet propagation, focusing on the magnetized flow dynamics and the mechanisms of energy conversion. We have developed the code L\'ostrego v1.0, a new 3D RMHD code to simulate astrophysical plasmas in Cartesian coordinates. Using this tool, we performed the first 3D RMHD numerical simulations of relativistic magnetized jets propagating through the clumpy stellar wind in a HMXB. The overall morphology and dynamics of weakly magnetized jet models is similar to previous hydrodynamical simulations, where the jet head generates a strong shock in the ambient medium and the initial over-pressure with respect to the stellar wind drives one or more recollimation shocks. In the time scales of our simulations, these jets are ballistic and seem to be more stable against internal instabilities than jets with the same power in the absence of fields. However, moderate-to-strong toroidal magnetic fields favour the development of current-driven instabilities and the disruption of the jet within the binary. A detailed analysis of the energy distribution in the relativistic outflow and the ambient medium reveals that both magnetic and internal energies can contribute to the effective acceleration of the jet. We certify that the jet feedback into the ambient medium is highly dependent on the jet energy distribution at injection, where hotter, more dilute and/or more magnetized jets are more efficient, as anticipated by feedback studies in the case of jets in active galaxies., Comment: 28 pages, 23 figures. Abstract shortened for arXiv. Accepted for publication in A&A

With the increased sensitivity and resolution of radio interferometry the study of the collimation and acceleration region of jets in Active Galactic Nuclei (AGN) has come into focus within the last years. Whereas a large fraction of AGN jets reveal a change from parabolic to conical collimation around the Bondi radius, a small number of sources deviate from this standard picture, including the radio galaxy NGC1052. We study the jet width profile, which provides valuable information about the interplay between the central engine and accretion disk system and the collimation and acceleration zone of the jets. We observed the double-sided active galaxy NGC1052 at six frequencies with the VLBA in 2017 and at 22GHz with RadioAstron in 2016. These data are combined with archival 15, 22, and 43 GHz multi-epoch VLBA observations. From ridge-line fitting we obtained width measurements along the jet and counter-jet which were fitted with broken power-laws. We find a break in the jet collimation profile at ~10^4 R_s (Schwarzschild radii). Downstream of the break the collimation is conical with a power-law index of 1.0 - 1.2 (cylindrical 0; parabolic 0.5; conical 1). The upstream power-law index of 0.36 for the approaching jet is neither cylindrical nor parabolic and for the receding jet with 0.16 close-to cylindrical. Both jets have an opening angle of ~30 degree at a distance of ~10^3 R_S and are well collimated with an opening angle of <10 degrees downstream of the break. There are significant differences in the upstream collimation profile between approaching (Eastern) and receding (Western) jet. Absorption or scattering in the surrounding torus as well as an accretion wind may mimic a cylindrical profile. We need to increase the observing frequencies, which do not suffer from absorption to find the true jet collimation profile upstream of 10^4 R_s., Comment: 14 pages, 12 figures. Accepted for publication in Astronomy & Astrophysics

We present RadioAstron space-based very long baseline interferometry (VLBI) observations of the nearby radio galaxy 3C84 (NGC1275) at the centre of the Perseus cluster. The observations were carried out on September 21-22, 2013 and involved a global array of 24 ground radio telescopes observing at 5 GHz and 22 GHz, together with the Space Radio Telescope (SRT). Furthermore, the Very Long Baseline Array (VLBA) and the phased Very Large Array (VLA) observed the source quasi-simultaneously at 15 GHz and 43 GHz. Fringes between the ground array and the SRT were detected on baseline lengths up to 8.1 times the Earth's diameter, providing unprecedented resolution for 3C 84 at these wavelengths. We note that the corresponding fringe spacing is 125 microarcsec at 5 GHz and 27 microarcsec at 22 GHz. Our space-VLBI images reveal a previously unseen sub-structure inside the compact 1 pc long jet that was ejected about ten years earlier. In the 5 GHz image, we detected, for the first time, low-intensity emission from a cocoon-like structure around the restarted jet. Our results suggest that the increased power of the young jet is inflating a bubble of hot plasma as it carves its way through the ambient medium of the central region of the galaxy. Here, we estimate the minimum energy stored in the mini-cocoon, along with its pressure, volume, expansion speed, and the ratio of heavy particles to relativistic electrons, as well as the density of the ambient medium. About half of the energy delivered by the jet is dumped into the mini-cocoon and the quasi-spherical shape of the bubble suggests that this energy may be transferred to a significantly larger volume of the interstellar medium than what would be accomplished by the well-collimated jet on its own. The pressure of the hot mini-cocoon also provides a natural explanation for the almost cylindrical jet profile seen in the 22 GHz RadioAstron image., Comment: To be published in Astronomy & Astrophysics

We performed multi-frequency studies on the gigahertz-peaked spectrum high-redshift quasar 0858-279. Initially, the source presented itself in the early VLBI images as a very peculiar resolved blob. We observed the quasar with the VLBA at 1.4-24 GHz in a dual-polarization mode. The high spatial resolution and the spectral index maps enabled us to resolve the core-jet structure and locate a weak and compact core by its inverted spectrum. The dominant jet component 20 parsecs away from the core was optically thin above 10 GHz and opaque below it. We also estimated an uncommonly strong magnetic field in the bright jet feature, which turned out to be around 1 G. The Faraday rotation measure maps revealed high RM values over 6000 rad/m^2. Additionally, these maps allowed us to follow the magnetic field direction in the bright jet feature being perpendicular to the propagation direction of the jet. All the results strongly indicated the formation of a shock wave in the dominant component arising from an interaction with the surrounding matter. Using the proposed hypothesis and the core shift approach, we discovered that the magnetic field in the core region is of the order of 0.1 G., Comment: 15 pages, 12 figures, accepted for publication in MNRAS

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, Comment: Accepted by A&A

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 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 multi-frequency and multi-scale 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, classified based on the accretion properties as low-excitation (LEG) and high-excitation (HEG) galaxies. Results.The jet collimation in NGC 315 is completed on sub-parsec scales. A transition from a parabolic to conical jet shape is detected at $z_{t}=0.58\pm0.28$ parsecs or ${\sim}5\times 10^3$ Schwarzschild radii ($R_{S}$) from the central engine, a distance which is much smaller than the Bondi radius, $r_{B}{\sim}92$ $\rm pc$, estimated based on X-ray data. The jet in this and in few other LEGs in our sample may be initially confined by a thick disk extending out to ${\sim}10^3$-$10^4$ $R_{S}$. A comparison between the mass-scaled jet expansion profiles of all sources indicates that jets in HEGs are surrounded by thicker disk-launched sheaths and collimate on larger scales with respect to jets in LEGs. These results suggest that disk winds play an important role in the jet collimation mechanism, particularly in high-luminosity sources. The impact of winds for the origin of the FRI/FRII dichotomy in radio galaxies is also discussed., Comment: 14 pages plus appendix, 6 figures. Accepted for publication in A&A

Flares in radio-loud AGN are thought to be associated with the injection of fresh plasma into the compact jet base. Such flares are usually strongest and appear earlier at shorter radio wavelengths. Hence, VLBI at mm-wavelengths is best suited to study the earliest structural changes of compact jets associated with emission flares. We study the morphological changes of the parsec-scale jet in the nearby (z=0.049) gamma-ray bright radio galaxy 3C111 following a flare that developed into a major radio outburst in 2007. We analyse three successive observations of 3C111 at 86 GHz with the Global mm-VLBI Array (GMVA) between 2007 and 2008 which yield a very high angular resolution of ~45muas. In addition, we make use of single-dish radio flux density measurements from the F-GAMMA and POLAMI programmes, archival single-dish and VLBI data. We resolve the flare into multiple plasma components with a distinct morphology resembling a bend in an otherwise remarkably straight jet. The flare-associated features move with apparent velocities of ~4.0c to ~4.5c and can be traced also at lower frequencies in later epochs. Near the base of the jet, we find two bright features with high brightness temperatures up to ~10^11K, which we associate with the core and a stationary feature in the jet. The flare led to multiple new jet components indicative of a dynamic modulation during the ejection. We interpret the bend-like feature as a direct result of the outburst which makes it possible to trace the transverse structure of the jet. In this scenario, the components follow different paths in the jet stream consistent with expectations for a spine-sheath structure, which is not seen during intermediate levels of activity. The possibility of coordinated multiwavelength observations during a future bright radio flare in 3C111 makes this source an excellent target for probing the radio-gamma-ray connection., Comment: 11 pages, 7 figures, accepted for publication by Astronomy & Astrophysics

Multi-wavelength spectral energy distributions of Low Mass X-ray Binaries in the hard state are determined by the emission from a jet, for frequencies up to mid-infrared, and emission from the accretion flow in the optical to X-ray range. In the last years, the flat radio-to-mid-IR spectra of Black Hole (BH) X-ray binaries was described using the internal shocks model, which assumes that the fluctuations in the velocity of the ejecta along the jet are driven by the fluctuations in the accretion flow, described by the X-ray Power Density Spectrum (PDS). In this work we attempt to apply this model for the first time to a Neutron Star (NS) LMXB, i.e. 4U 0614+091. We used the multi-wavelength data set obtained in 2006, comprising data from radio to X-ray, and applied a model which includes an irradiated disc model for the accretion flow and an updated version of the internal shocks code for the ejection. The new version of the code allows to change the geometry of the jet for the case of non-conical jets. Only two alternative scenarios provide a satisfactory description of the data: using the X-ray PDS but in a non-conical geometry for the jet, or either using a conical geometry but with a "flicker-noise" PDS. Both scenarios would imply some differences with the results obtained with similar models on BH X-ray binaries, shedding light on the possibility that jets in NS and BH binaries might somehow have a different geometry or a different coupling with the accretion flow., Comment: 18 pages, 12 figures. Accepted for publication by MNRAS

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. We have performed target-of-opportunity VLBI imaging observations at 43 GHz frequency with the VLBA two and eight months, respectively, after the neutrino event. We produced two images of TXS 0506+056 with angular resolutions of (0.2x1.1) mas and (0.2x0.5) 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 about 0.5 mas from the mm-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. The core expands in size with apparent superluminal velocity during these six months so that the brightness temperature drops by a factor of three in spite of the strong flux density increase. 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 mm-VLBI core. This structure is consistent with theoretical models that attribute the neutrino and {\gamma}-ray production 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, Comment: Accepted for publication in Astronomy & Astronomy Letters; 5 pages, 3 figures, 2 tables

Detailed studies of relativistic jets in active galactic nuclei (AGN) require high-fidelity imaging at the highest possible resolution. This can be achieved using very long baseline interferometry (VLBI) at radio frequencies, combining worldwide (global) VLBI arrays of radio telescopes with a space-borne antenna on board a satellite. We present multiwavelength images made of the radio emission in the powerful quasar S5 0836+710, obtained using a global VLBI array and the antenna Spektr-R of the RadioAstron mission of the Russian Space Agency, with the goal of studying the internal structure and physics of the relativistic jet in this object. The RadioAstron observations at wavelengths of 18cm, 6cm, and 1.3cm are part of the Key Science Program for imaging radio emission in strong AGN. The internal structure of the jet is studied by analyzing transverse intensity profiles and modeling the structural patterns developing in the flow. The RadioAstron images reveal a wealth of structural detail in the jet of S5 0836+710 on angular scales ranging from 0.02mas to 200mas. Brightness temperatures in excess of $10^{13}$\,K are measured in the jet, requiring Doppler factors of $\ge 100$ for reconciling them with the inverse Compton limit. Several oscillatory patterns are identified in the ridge line of the jet and can be explained in terms of the Kelvin-Helmholtz (KH) instability. The oscillatory patterns are interpreted as the surface and body wavelengths of the helical mode of the KH instability. The interpretation provides estimates of the jet Mach number and of the ratio of the jet to the ambient density, which are found to be $M_\mathrm{j}\approx 12$ and $\eta\approx 0.33$. The ratio of the jet to the ambient density should be conservatively considered an upper limit because its estimate relies on approximations., Comment: 16 pages, 9 figures (9 extra figures in Appendix), in press; Astronomy and Astrophysics

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. 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. 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. 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-to-ambient medium density ratio and Mach number. We obtain a jet Lorentz factor $\gamma \simeq 12$, specific internal energy of $\varepsilon \simeq 10^{-2}\,c^2$, jet-to-ambient medium density ratio of $\eta\approx 10^{-3}$, and an internal (classical) jet Mach number of $M_\mathrm{j}\approx 12$. We also find that the wavelength ratios are better recovered by a transversal structure with a width of $\simeq 10\,\%$ of the jet radius. This method represents a powerful tool to derive the jet parameters in all jets showing helical patterns with different wavelengths., Comment: Accepted for publication in A&A, 15 pages, 12 figures

High-resolution Very-Long-Baseline Interferometry observations of NGC 1052 show a two sided jet with several regions of enhanced emission and a clear emission gap between the two jets.This gap shrinks with increasing frequency and vanishes around $\nu\sim43$ GHz. The observed structures are due to both the macroscopic fluid dynamics interacting with the surrounding ambient medium including an obscuring torus and the radiation microphysics. In this paper we investigate the possible physical conditions in relativistic jets of NGC 1052 by directly modelling the observed emission and spectra via state-of-the-art special-relativistic hydrodynamic (SRHD) simulations and radiative transfer calculations. To investigate the physical conditions in the relativistic jet we coupled our radiative transfer code to evolutionary algorithms and performed simultaneous modelling of the observed jet structure and the broadband radio spectrum. During the calculation of the radiation we consider both thermal and non-thermal emission. In order to compare our model to VLBI observations we take into account the sparse sampling of the u-v plane, the array properties and the imaging algorithm. We present for the first time an end-to-end pipeline for fitting numerical simulations to VLBI observations of relativistic jets taking into account the macrophysics including fluid dynamics and ambient medium configurations together with thermal/non-thermal emission and the properties of the observing array. The detailed analysis of our simulations shows that the structure and properties of the observed relativistic jets in NGC 1052 can be reconstructed by a slightly over-pressured jet ($d_k\sim1.5$) embedded in a decreasing pressure ambient medium, Comment: submitted to A&A, 24 pages, 25 figures

Context: Few Active Galactic Nuclei (AGN) reveal a double sided jet system. However, these are crucial to understand basic physical properties of extragalactic jets. Aims: We address the questions whether jets in AGN are symmetric in nature, how well they are collimated on small scales, and how do they evolve with time. Methods: We monitored the sub-parsec scale morphology of NGC1052 with the VLBA at 43GHz from 2005 to 2009. Results: A detailed study of 29 epochs show an asymmetry between both jets. A kinematic analysis of the outflows reveals higher apparent velocities for the eastern jet as compared to the western one: beta_ej = 0.529+-0.038 and beta_wj = 0.343+-0.037, respectively. Contradictory to previous studies, we find higher flux densities for the western jet. The distribution of brightness temperature and jet width features well collimated jets up to 1mas distance to the dynamic center and a nearly conical outflow further outwards. Combining flux density ratios and velocities of the jet flow, we were not able to find a combination of intrinsic velocity and inclination angle of the jets that is consistent for all observations, which is contradictory to symmetrically evolving jets. Spectral index maps between quasi-simultaneous 22GHz and 43GHz observations support the existence of an optically thick absorber covering the innermost 1.6 mas around the 43GHz central feature and an optically thin jet emission with a spectral index of <= -1. Conclusions: Our results fit into a picture in which we expect larger internal energy and/or magnetic flux in the western jet and higher kinetic energy in the eastern jet. Previous observations at lower frequencies found slower velocities of the moving jet features as compared to this work. Considering the different velocities at different areas we suggest a spine-sheath structure with a faster inner layer and a slower outer layer., Comment: Accepted by Astronomy & Astrophysics

We aim to better understand the dynamics within relativistic magneto-hydrodynamical flows in the extreme environment and close vicinity of supermassive black holes. To do so, we analyze the peculiar radio galaxy 3C 111, for which long-term polarimetric observations are available. We make use of the high spatial resolution of the VLBA network and the MOJAVE monitoring program, which provides high data quality also for single sources and allows us to study jet dynamics on parsec scales in full polarization with an evenly sampled time-domain. We additionally consider data from the IRAM 30-m Telescope as well as the SMA. Jet properties such as the electric vectors, the (polarized) flux density, feature size, and brightness temperature, describe a complex evolution of the polarized jet. The electric vector position angles (EVPAs) of features traveling down the jet perform a large and smooth rotation of $\gtrsim 180^{\circ}$ across a distance of about 20 pc. In contrast, the EVPAs are strongly variable within the first parsecs of the jet. We find a tendency towards transverse EVPAs across the jet with a local anomaly of aligned vectors in between. The transverse extent of the flow decreases coincident with a jump in brightness temperature around where we observe the EVPAs to turn into alignment with the jet flow. Also the gradients of the feature size and particle density with distance steepen in that region. We interpret the propagating polarized features with shocks and the observed local anomalies with the interaction of these shocks with a recollimation shock of the underlying flow. Together with a sheared magnetic field, this shock-shock interaction can explain the large rotation of the EVPA. The superimposed variability of the EVPAs close to the core is likely related to a clumpy Faraday screen, which also contributes significantly to the observed EVPA rotation in that region., Comment: 19 pages, 22 figures, 2 tables, accepted for publication by A&A

High-resolution Very-Long-Baseline Interferometry observations of active galactic nuclei have revealed asymmetric structures in the jets of radio galaxies. These asymmetric structures may be due to internal asymmetries in the jet, could be induced by the different conditions in the surrounding ambient medium including the obscuring torus, or a combination of the two. In this paper we investigate the influence of the ambient medium (including the obscuring torus) on the observed properties of jets from radio galaxies. We performed special-relativistic hydrodynamic (RHD) simulations of over-pressured and pressure-matched jets using the special-relativistic hydrodynamics code \texttt{Ratpenat}, which is based on a second-order accurate finite-volume method and an approximate Riemann solver. Using a newly developed emission code to compute the electromagnetic emission, we have investigated the influence of different ambient medium and torus configurations on the jet structure and subsequently computed the non-thermal emission produced by the jet and the thermal absorption due to the torus. To better compare the emission simulations with observations we produced synthetic radio maps, taking into account the properties of the observatory. The detailed analysis of our simulations shows that the observed asymmetries can be produced by the interaction of the jet with the ambient medium and by the absorption properties of the obscuring torus., Comment: 14 pages, 17 figures, submitted to A&A

Supermassive black holes (SMBH) are essential for the production of jets in radio-loud active galactic nuclei (AGN). Theoretical models based on Blandford & Znajek extract the rotational energy from a Kerr black hole, which could be the case for NGC1052, to launch these jets. This requires magnetic fields of the order of $10^3\,$G to $10^4\,$G. We imaged the vicinity of the SMBH of the AGN NGC1052 with the Global Millimetre VLBI Array and found a bright and compact central feature, smaller than 1.9 light days (100 Schwarzschild radii) in radius. Interpreting this as a blend of the unresolved jet bases, we derive the magnetic field at 1 Schwarzschild radius to lie between 200 G and ~80000 G consistent with Blandford & Znajek models., Comment: 8 pages, 7 figures, accepted by Astronomy & Astrophysics

High resolution Very Long Baseline Interferometry (VLBI) observations of Active Galactic Nuclei (AGN) revealed traveling and stationary or quasi-stationary radio-components in several blazar jets. The traveling ones are in general interpreted as shock waves generated by pressure perturbations injected at the jet nozzle. The stationary features can be interpreted as recollimation shocks in non-pressure matched jets if they show a quasi-symmetric bump in the spectral index distribution. In some jets there may be interactions between the two kinds of shocks. These shock--shock interactions are observable with VLBI techniques, and their signature should also be imprinted on the single--dish light curves. We performed relativistic hydrodynamic (RHD) simulations of over-pressured and pressure-matched jets. To simulate the shock interaction we injected a perturbation at the jet nozzle once a steady-state was reached. We computed the non-thermal emission (including adiabatic and synchotron losses) resulting from the simulation. We show that the injection of perturbations in a jet can produce a bump in emission at GHz frequencies previous to the main flare, which is produced when the perturbation fills the jet in the observer's frame. The detailed analysis of our simulations and the non-thermal emission calculations show that interaction between a recollimation shock and traveling shock produce a typical and clear signature in both the single--dish light curves and in the VLBI observations: the flaring peaks are higher and delayed with respect to the evolution of a perturbation through a conical jet. This fact can allow to detect such interactions for stationary components lying outside of the region in where the losses are dominated by inverse Compton scattering., Comment: 15 pages, 17 figures

The Square Kilometre Array (SKA) is called to revolutionise essentially all areas of Astrophysics. With a collecting area of about a square kilometre, the SKA will be a transformational instrument, and its scientific potential will go beyond the interests of astronomers. Its technological challenges and huge cost requires a multinational effort, and Europe has recognised this by putting the SKA on the roadmap of the European Strategy Forum for Research Infrastructures (ESFRI). The Spanish SKA White Book is the result of the coordinated effort of 120 astronomers from 40 different research centers. The book shows the enormous scientific interest of the Spanish astronomical community in the SKA and warrants an optimum scientific exploitation of the SKA by Spanish researchers, if Spain enters the SKA project., Comment: Editors: Miguel A. Perez-Torres (Editor-in-chief), L. Verdes-Montenegro, J. C. Guirado, A. Alberdi, J. Martin-Pintado, R. Bachiller, D. Herranz, J. M. Girart, J. Gorgas, C. Hernandez-Monteagudo, S. Migliari and J. M. Rodriguez Espinosa; 289 pages, published in June 2015 by Sociedad Espa\~nola de Astronom\'ia

The jet-counterjet system of the closest radio-loud active galaxy Centaurus A (Cen A) can be studied with Very Long Baseline Interferometry (VLBI) on unprecedented small linear scales of ~0.018pc. These high-resolution observations provide essential information on jet emission and propagation within the inner parsec of an AGN jet. We present the results of a kinematic study performed within the framework of the Southern-hemisphere AGN monitoring program TANAMI. Over 3.5years, the evolution of the central-parsec jet structure of Cen A was monitored with VLBI. These observations reveal complex jet dynamics which are well explained by a spine-sheath structure supported by the downstream acceleration occurring where the jet becomes optically thin. Both moving and stationary jet features are tracked. A persistent local minimum in surface brightness suggests the presence of an obstacle interrupting the jet flow, which can be explained by the interaction of the jet with a star at a distance of ~0.4pc from the central black hole. We briefly discuss possible implications of such an interaction regarding the expected neutrino and high-energy emission and the effect on a putative planet., Comment: 6 pages, proceedings of the 12th European VLBI Network Symposium and Users Meeting - EVN 2014, 7-10 October 2014, Cagliari, Italy. Published online in PoS, ID.024

Blazars are among the most powerful extragalactic objects, as a sub-class of active galactic nuclei. They launch relativistic jets and their emitted radiation shows strong variability across the entire electro-magnetic spectrum. The mechanisms producing the variability are still controversial and different models have been proposed to explain the observed variations in multi-frequency blazar light curves.We investigate the capabilities of the classical shock-in-jet model to explain and reconstruct the observed evolution of flares in the turnover frequency turnover flux density plane and their frequency-dependent light curve parameters. With a detailed parameter space study we provide the framework for future, detailed comparisons of observed flare signatures with the shock-in-jet scenario. Based on the shock model we compute synthetic single-dish light curves at different radio frequencies (2.6 to 345 GHz) and for different physical conditions in a conical jet (e.g. magnetic field geometry and Doppler factor). From those we extract the slopes of the different energy loss stages within the $\nu_\mathrm{m}$-$S_\mathrm{m}$ plane and deduce the frequency-dependence of different light curve parameters such as flare amplitude, time scale and cross-band delays. The evolution of the Doppler factor along the jet has the largest influence on the evolution of the flare and on the frequency-dependent light curve parameters. The synchrotron stage can be hidden in the Compton or in the adiabatic stage, depending mainly on the evolution of the Doppler factor, which makes it difficult to detect its signature in observations. In addition, we show that the time lags between different frequencies can be used as an efficient tool to better constrain the physical properties of these objects., Comment: 21 pages, 17 figures

Relativistic jets in active galactic nuclei represent one of the most powerful phenomena in the Universe. They form in the surroundings of the supermassive black holes as a by-product of accretion onto the central black hole in active galaxies. The flow in the jets propagates at velocities close to the speed of light. The distance between the first part of the jet that is visible in radio images (core) and the black hole is still a matter of debate. Only very-long-baseline interferometry observations resolve the innermost compact regions of the radio jet. Those can access the jet base, and combining data at different wavelenghts, address the physical parameters of the outflow from its emission. We have performed an accurate analysis of the frequency-dependent shift of the VLBI core location for a multi-wavelength set of images of the blazar CTA 102 including data from 6 cm down to 3 mm. The measure of the position of the central black hole, with mass $\sim 10^{8.93}\,M_\odot$, in the blazar CTA 102 reveals a distance of $\sim 8\times10^4$ gravitational radii to the 86 GHz core, in agreement with similar measures obtained for other blazars and distant radio galaxies, and in contrast with recent results for the case of nearby radio galaxies, which show distances between the black hole and the radio core that can be two orders of magnitude smaller., Comment: 4 pages, 2 figures

(abridged)The structure formed by the shocked winds of a massive star and a non-accreting pulsar in a binary suffers periodic and random variations of orbital and non-linear dynamical origin. For the 1st time, we simulate in 3 D the interaction of isotropic stellar and relativistic pulsar winds along 1 full orbit, on scales well beyond the binary size. We also investigate the impact of grid resolution and size, and of different EoOs: a gamma-constant ideal gas, and an ideal gas with gamma dependent on temperature. We carry out, with the code PLUTO, relativistic HD simulations in 2 and 3 D of the interaction of a slow wind and a relativistic wind with Gamma=2 along 1 full orbit up to ~100 x the binary size. The different 2-D simulations are carried out with equal and larger grid resolution and size, and 1 of them is done with a more realistic equation of state, than in 3 D. The simulations in 3 D confirm previous results in 2 D. The shocked flows are subject to a faster instabilities growth in 3 D, which enhances the presence of shocks, mixing, and large-scale disruption. In 2 D, higher resolution simulations confirm lower resolution results, simulations with larger grid sizes strengthen the case for the loss of global coherence of the shocked-wind structure, and simulations with 2 different EoOs yield very similar results. In addition to the KHI, we find that the Richtmyer-Meshkov and the RTI are likely acting together in the shocked flow evolution. Simulations in 3 D confirm that the interaction of stellar and pulsar winds yields structures that evolve non-linearly and get strongly entangled. The evolution is accompanied by strong kinetic energy dissipation, rapid changes in flow orientation and speed, and turbulent motion. The results strengthen the case for the loss of global coherence of the shocked structure on large scales, although higher pulsar wind speed simulations are needed., Comment: 13 pages, 12 figures, accepted for publication in Astronomy and Astrophysics

Centaurus A is the closest radio-loud active galaxy. Very Long Baseline Interferometry (VLBI) enables us to study the jet-counterjet system on milliarcsecond (mas) scales, providing essential information for jet emission and propagation models. We study the evolution of the central parsec jet structure of Cen A over 3.5 years. The proper motion analysis of individual jet components allows us to constrain jet formation and propagation and to test the proposed correlation of increased high energy flux with jet ejection events. Cen A is an exceptional laboratory for such detailed study as its proximity translates to unrivaled linear resolution, where 1 mas corresponds to 0.018 pc. The first 7 epochs of high-resolution TANAMI VLBI observations at 8 GHz of Cen A are presented, resolving the jet on (sub-)mas scales. They show a differential motion of the sub-pc scale jet with significantly higher component speeds further downstream where the jet becomes optically thin. We determined apparent component speeds within a range of 0.1c to 0.3c, as well as identified long-term stable features. In combination with the jet-to-counterjet ratio we can constrain the angle to the line of sight to ~12{\deg} to 45{\deg}. The high resolution kinematics are best explained by a spine-sheath structure supported by the downstream acceleration occurring where the jet becomes optically thin. On top of the underlying, continuous flow, TANAMI observations clearly resolve individual jet features. The flow appears to be interrupted by an obstacle causing a local decrease in surface brightness and a circumfluent jet behavior. We propose a jet-star interaction scenario to explain this appearance. The comparison of jet ejection times with high X-ray flux phases yields a partial overlap of the onset of the X-ray emission and increasing jet activity, but the limited data do not support a robust correlation., Comment: 16 pages, 10 figures, A&A, 569, L115 (accepted 23th June, published online 1st October)

Very long baseline interferometry at millimetre/submillimetre wavelengths (mmVLBI) offers the highest achievable spatial resolution at any wavelength in astronomy. The anticipated inclusion of ALMA as a phased array into a global VLBI network will bring unprecedented sensitivity and a transformational leap in capabilities for mmVLBI. Building on years of pioneering efforts in the US and Europe the ongoing ALMA Phasing Project (APP), a US-led international collaboration with MPIfR-led European contributions, is expected to deliver a beamformer and VLBI capability to ALMA by the end of 2014 (APP: Fish et al. 2013, arXiv:1309.3519). This report focuses on the future use of mmVLBI by the international users community from a European viewpoint. Firstly, it highlights the intense science interest in Europe in future mmVLBI observations as compiled from the responses to a general call to the European community for future research projects. A wide range of research is presented that includes, amongst others: - Imaging the event horizon of the black hole at the centre of the Galaxy - Testing the theory of General Relativity an/or searching for alternative theories - Studying the origin of AGN jets and jet formation - Cosmological evolution of galaxies and BHs, AGN feedback - Masers in the Milky Way (in stars and star-forming regions) - Extragalactic emission lines and astro-chemistry - Redshifted absorption lines in distant galaxies and study of the ISM and circumnuclear gas - Pulsars, neutron stars, X-ray binaries - Testing cosmology - Testing fundamental physical constants, Comment: Replaced figures 2 and 3: corrected position SRT. Corrected minor typo in 5.1

The dynamics and stability of extragalactic jets may be strongly influenced by small (and probable) differences in pressure between the jet and the ambient and within the jet itself. The former give rise to expansion and recollimation of the jet. This occurs in the form of conical shocks, or Mach disks, if the pressure difference is large enough. Pressure asymmetries within the jet may trigger the development of helical patterns via coupling to kink current-driven instability, or to helical Kelvin-Helmholtz instability, depending on the physical conditions in the jet. I summarize here the evidence collected during the last years on the presence of recollimation shocks and waves in jets. In the jet of CTA 102 evidence has been found for (traveling)shock-(standing)shock interaction in the core-region (0.1 mas from the core), using information from the light-curve of the source combined with VLBI data. The conclusions derived have been confirmed by numerical simulations combined with emission calculations that have allowed to study the spectral evolution of the perturbed jet. Helical structures can also be identified in radio-jets. The ridge-line of emission of the jet of S5~0836+710 has been identified as a physical structure corresponding to a wave developing in the jet flow. I review here the evidence that has allowed to reach this conclusion, along with an associated caveat. Current data do not allow to distinguish between magnetic or hydrodynamical instabilities. I finally discuss the importance of these linear and non-linear waves for jet evolution., Comment: 11 pages. Proceedings of the conference: The innermost regions of relativistic jets and their magnetic fields. Corrected typos and added references

The temporal and spatial spectral evolution of the jets of AGN can be studied with multi-frequency, multi-epoch VLBI observations. The combination of both, morphological and spectral parameters can be used to derive source intrinsic physical properties such as the magnetic field and the non-thermal particle density. In the first two papers of this series, we analyzed the single-dish light curves and the VLBI kinematics of the blazar CTA 102 and suggested a shock-shock interaction between a traveling and a standing shock wave as a possible scenario to explain the observed evolution of the component associated to the 2006 flare. In this paper we investigate the core-shift and spectral evolution to test our hypothesis of a shock-shock interaction. We used 8 multi-frequency VLBA observations to analyze the temporal and spatial evolution of the spectral parameters during the flare. We observed CTA 102 between May 2005 and April 2007 using the VLBA at six different frequencies spanning from 2 - 86 GHz. After the calibrated VLBA images were corrected for opacity, we performed a detailed spectral analysis. From the derived values we estimated the magnetic field and the density of the relativistic particles. The detailed analysis of the opacity shift reveals that the position of the jet core is proportional to nu^-1 with some temporal variations. The value suggests possible equipartition between magnetic field energy and particle kinetic energy densities at the most compact regions. From the variation of the physical parameters we deduced that the 2006 flare in CTA 102 is connected to the ejection of a new traveling feature (t=2005.9) and the interaction between this shock wave and a stationary structure around 0.1 mas from the core. The source kinematics together with the spectral and structural variations can be described by helical motions in an over-pressured jet., Comment: 35 pages, 46 figures

The 2006 radio flare in the blazar CTA102 is the largest ever reported for this source. The analysis of the single--dish light curves revealed a possible shock-shock interaction as a scenario for the 2006 outburst. In order to confirm this hypothesis we analyzed eight multi-frequency (2 GHz - 86 GHz) VLBI observations covering the 2006 flare. We detected a radio component ejected around 2005.9 and a stationary feature around 0.1 mas away from the core. We also performed a core-shift and spectral analysis on the fully calibrated VLBI maps. The frequency dependent position of the core slightly deviates from the typical $\nu^{-1}$-dependence during the flaring state. This behaviour may reflect a mismatch between magnetic energy density and energy density of the relativistic particles. From the core-shift corrected maps we obtained the spectral parameters and computed the temporal and spatial evolution of the jet intrinsic parameters such as the magnetic field and the particle density. The overall picture of the source could be best interpreted as an over-pressured jet perturbed by traveling shock waves., Comment: 6 pages, 3 figures, Proceedings of the 11th EVN Symposium. Bordeaux (France), October 2012

We make use of VLBI observations of the radio jet in the quasar S5 0836+710 at different frequencies and epochs to study its properties. The jet shows helical structure at all frequencies. The ridge-line of the emission in the jet coincides at all frequencies and epochs, within the errors. We conclude that the helicity is a real, physical structure. Small differences between epochs reveal wave-like motion of the ridge-line transversal to the jet propagation axis. These transversal motions are measured to be superluminal. This unphysical result could correspond to a possible small amplitude oscillation of the ridge-line at the radio-core and to large errors in the determination of the positions. In addition, higher resolution images at 15 GHz show that the ridge-line does not coincide exactly with the centre of the radio jet. At arc-second scales, this powerful jet shows non-collimated, irregular structure and a lack of a hot-spot. Following this collection of evidence, we conclude that the ridge-line could be related to a pressure maximum within the jet cross-section, associated with the observed helical pattern that could lead to jet disruption at longer scales. We discuss the implications regarding the nature of the instability and the jet flow itself., Comment: 6 pages. Proceedings of the 11th EVN Symposium. Bordeaux (France), October 2012

We performed multifrequency multiepoch Very Long Baseline Array (VLBA) observations of the blazar CTA 102 during its 2006 radio flare, the strongest ever reported for this source. These observations provide an excellent opportunity to investigate the evolution of the physical properties of blazars, especially during these flaring events. We want to study the kinematic changes in the source during the strong radio outburst in April 2006 and test the assumption of a shock-shock interaction. This assumption is based on the analysis and modeling of the single-dish observations of CTA\,102 (Paper I). In this paper we study the kinematics of CTA 102 at several frequencies using VLBI observations. From the modeled jet features we derived estimates for the evolution of the physical parameters, such as the particle density and the magnetic field. Furthermore, we combined our observations during the 2006 flare with long-term VLBA monitoring of the source at 15 GHz and 43 GHz. We cross-identified seven features throughout our entire multifrequency observations and find evidence of two possible recollimation shocks around 0.1 mas (deprojected 18 pc at a viewing angle of 2.6 degrees) and 6.0 mas (deprojected 1 kpc) from the core. The 43 GHz observations reveal a feature ejected at epoch $t_\mathrm{ej}=2005.9\pm0.2$, which could be connected to the 2006 April radio flare. Furthermore, this feature might be associated with the traveling component involved in the possible shock-shock interaction, which gives rise to the observed double peak structure in the single-dish light curves reported in Paper I., Comment: 34 pages, 29 figures, A\& A accepted

The MOJAVE survey contains 101 quasars with a total of 354 observed radio components that are different from the radio cores, among which 95% move with apparent projected superluminal velocities with respect to the core, and 45% have projected velocities larger than 10c (with a maximum velocity 60c). Doppler boosting effects are analyzed to determine the statistics of the superluminal motions. We integrate over all possible values of the Lorentz factor the values of the kinetic energy corresponding to each component. The calculation of the mass in the ejection is carried out by assuming the minimum energy state. This kinetic energy is multiplied by the frequency at which the portions of the jet fluid identified as "blobs" are produced. Hence, we estimate the average total power released by the quasars in the form of kinetic energy in the long term on pc-scales. RESULTS. A selection effect in which both the core and the blobs of the quasar are affected by huge Doppler-boosting enhancement increases the probability of finding a jet ejected within 10 degrees of the line of sight >~40 times above what one would expect for a random distribution of ejection, which explains the ratios of the very high projected velocities given above. The average total kinetic power of each MOJAVE quasar should be very high to obtain this distribution: ~7E47 erg/s. This amount is much higher than previous estimates of kinetic power on kpc-scales based on the analysis of cavities in X-ray gas or radio lobes in samples of objects of much lower radio luminosity but similar black hole masses. The kinetic power is a significant portion of the Eddington luminosity, on the order of the bolometric luminosity, and proportional on average to square root of the radio luminosity, although this correlation might be induced by Malmquist-like bias., Comment: Published in A&A; v2: minor changes to fit the published version

The F-GAMMA program is among the most comprehensive programs that aim at understanding the physics in active galactic nuclei through the multi-frequency monitoring of Fermi blazars. Here we discuss monthly sampled broad-band radio spectra (2.6 - 142 GHz). Two different studies are presented. (a) We discuss that the variability patterns traced can be classified into two classes: (1) to those showing intense spectral-evolution and (2) those showing a self-similar quasi-achromatic behaviour. We show that a simple two-component model can very well reproduce the observed phenomenologies. (b) We present the cm-to-mm behaviour of three gamma-ray bright Narrow Line Seyfert 1 galaxies over time spans varying between ~1.5 and 3 years and compare their variability characteristics with typical blazars., Comment: 2012 Fermi & Jansky Proceedings - eConf C1111101

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. 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. 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. 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. 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., Comment: 8 pages, 11 figures, Astronomy and Astrophysics, in press, minor changes after acceptance

The F-GAMMA program is a coordinated effort to investigate the physics of Active Galactic Nuclei (AGNs) via multi-frequency monitoring of Fermi blazars. In the current study we show and discuss the evolution of broad-band radio spectra, which are measured at ten frequencies between 2.64 and 142 GHz using the Effelsberg 100-m and the IRAM 30-m telescopes. It is shown that any of the 78 sources studied can be classified in terms of their variability characteristics in merely 5 types of variability. It is argued that these can be attributed to only two classes of variability mechanisms. The first four types are dominated by spectral evolution and can be described by a simple two-component system composed of: (a) a steep quiescent spectral component from a large scale jet and (b) a time evolving flare component following the "Shock-in-Jet" evolutionary path. The fifth type is characterised by an achromatic change of the broad band spectrum, which could be attributed to a different mechanism, likely involving differential Doppler boosting caused by geometrical effects. Here we present the classification, the assumed physical scenario and the results of calculations that have been performed for the spectral evolution of flares., Comment: Proceedings of the conference: "The Central Kiloparsec in Galactic Nucleic: Astronomy at High Angular Resolution 2011", August 29 - September 2, 2011, Bad Honnef, Germany

High energy emission can be produced in the interaction sites of both galactic and extragalactic jets with the surrounding medium. We have developed a radiative model that accounts for the continuous injection of relativistic electrons in the forward, reverse and recollimation shocks developed in the shell, cocoon and reconfinement interaction regions, respectively. We also performed hydrodynamical simulations to establish the physical properties in both galactic and extragalactic systems. The resulting non-thermal emission is studied assuming different values for the jet power, the external mass density and the source age for both FR I galaxies and galactic microquasars. The obtained fluxes are compared to current instrument sensitivities at radio, X-ray and gamma-ray bands., Comment: Proceedings of the 8th INTEGRAL Workshop: The Restless Gamma-ray Universe 2010

The Compact Symmetric Objects (CSOs) are small (<1 kiloparsec) and powerful extragalactic radio sources showing emission on both sides of an active galactic nucleus and no signs of strong relativistic beaming. They may be young radio sources, progenitors of large FRII radio galaxies. We aim to study the statistical properties of CSOs by constructing and investigating a new large sample of CSO candidates on the basis of dual-frequency, parsec-scale morphology. For the candidate selection we utilized VLBI data for 4170 extragalactic objects obtained simultaneously at 2.3 and 8.6 GHz (S and X band) within the VLBA Calibrator Survey 1-6 and the Research and Development - VLBA projects. Properties of their broad-band radio spectra were characterized by using RATAN-600 observations. Numerical modeling was applied in an attempt to explain the observed effects. A sample of 64 candidate CSOs is identified. The median two-point S-X band spectral index of parsec-scale hotspots is found to be -0.52; with the median brightness temperature ~10^9 K at X band. Statistical analysis reveals a systematic difference between positions of brightest CSO components (associated with hotspots) measured in the S and X bands. The distance between these components is found to be on average 0.32+/-0.06 mas greater at 8.6 GHz than at 2.3 GHz. This difference in distances cannot be explained by different resolutions at the S and X bands. It is a manifestation of spectral index gradients across CSO components, which may potentially provide important physical information about them. Despite our detailed numerical modeling of a CSO hotspot, the model was not able to reproduce the magnitude of the observed positional difference. A more detailed modeling may shed light on the origin of the effect., Comment: 34 pages, 8 figures, 2 tables, accepted to A&A

Context: The blazar CTA 102 (z=1.037) underwent a historical radio outburst in April 2006. This event offered a unique chance to study the physical properties of the jet. Aims: We used multifrequency radio and mm observations to analyze the evolution of the spectral parameters during the flare as a test of the shock-in-jet model under these extreme conditions. Methods: For the analysis of the flare we took into account that the flaring spectrum is superimposed on a quiescent spectrum. We reconstructed the latter from archival data and fitted a synchrotron self-absorbed distribution of emission. The uncertainties of the derived spectral parameters were calculated using Monte Carlo simulations. The spectral evolution is modeled by the shock-in-jet model, and the derived results are discussed in the context of a geometrical model (varying viewing angle) and shock-shock interaction. Results: The evolution of the flare in the turnover frequency-turnover flux density plane shows a double peak structure. The nature of this evolution is dicussed in the frame of shock-in-jet models. We discard the generation of the double peak structure in the turnover frequency-turnover flux density plane purely based on geometrical changes (variation of the Doppler factor). The detailed modeling of the spectral evolution favors a shock-shock interaction as a possible physical mechanism behind the deviations from the standard shock-in-jet model., Comment: 15 pages, 12 figures

The environment of high-mass X-ray binaries can be characterized either by the SNR that forms these systems, or by the wind from the companion massive star. These regions should be tenuous but very hot, and surrounded by a dense and cold shocked ISM shell. The interaction between the jet and such a complex medium, also affected by the system proper motion, can lead to very different jet termination structures. The evolution of the jet termination regions during the life of a high-mass microquasar is simulated to improve the present understanding of these structures. Also, the evolving emission characteristics are modeled to inform potential observational campaigns for this class of object. We have performed 2D numerical simulations of jets propagating in different scenarios, corresponding to different epochs after the formation of the high-mass X-ray binary, using the code Ratpenat. We have also made simple estimates of the non-thermal emission that could be produced in the jet termination regions. We find that, in the way through the hot and tenuous medium of the shocked wind/SNR ejecta, the jet suffers recollimation shocks in which it loses part of its thrust and ends in a strong shock inflating a hot cocoon. The jet head propagates with a speed similar to the medium sound speed, until it eventually reaches the denser and colder shocked ISM and the unperturbed ISM later on. In these last stages of evolution, the jet is significantly slowed down and can be disrupted. For relatively old sources, the microquasar peculiar velocity becomes important, leading to complete jet destruction. Extended non-thermal radiation can be generated in the jet termination regions, being hard X-rays and TeV photons the best suited wavelengths to observe these structures., Comment: 11 pages, 15 Figures, 2 Tables, Astronomy & Astrophysics, in press

The blazar CTA 102 underwent a major radio flare in April 2006. We used several 15 GHz VLBI observations from the MOJAVE program to investigate the influence of this extreme event on jet kinematics. The result of modeling and analysis lead to the suggestion of an interaction between traveling and standing shocks 0.2 mas away from the VLBI core., Comment: Proceedings of the meeting "Steady and Transient Jets", MPIfR Bonn (2010) 2 figures, 4-5 pages

The radio light curve and spectral evolution of the blazar CTA 102 during its 2006 outburst can be rather well explained by the standard shock-in-jet model. The results of a pixel-to-pixel spectral analysis of multi-frequency VLBI images, together with kinematics derived from the MOJAVE survey lead to the picture of an over-pressured jet with respect to the ambient medium. The interaction of a traveling shock wave with a standing one is a possible scenario which could explain the observed spectral behaviour, Comment: Proceedings of the meeting "Fermi meets Jansky - AGN at Radio and Gamma-Rays", edited by T. Savolainen, E. Ros, R.W. Porcas, J.A. Zensus, MPIfR, Bonn (2010) 5 figures, 4-5 pages, needs fmj2010.cls

We have found evidence for interaction between a standing and a traveling shock in the jet of the blazar CTA 102. Our result is based in the study of the spectral evolution of the turnover frequency-turnover flux density plane. The radio/mm light curves were taken during a major radio outburst in April 2006., Comment: Proceedings of IAU Symposium No. 275: "Jets at all Scales", edited by G. E. Romero, R. A. Sunyaev, T. Belloni, Buenos Aires (2010) no figures, 2 pages

We present the first three-dimensional simulations of the evolution of a microquasar jet inside the binary-star system. The aim is to study the interaction of these jets with the stellar wind from a massive companion and the possible locations of high-energy emission sites. We have simulated two jets with different injection power in order to give a hint on the minimum power required for the jet to escape the system and become visible in larger scales. In the setup, we include a massive star wind filling the grid through which the jet evolves. We show that jets should have powers of the order of $10^{37}\rm{erg/s}$ or more in order not to be destroyed by the stellar wind. The jet-wind interaction results in regions in which high energy emission could be produced. These results imply the possible existence of a population of X-ray binaries not detected in the radio band due to jet disruption inside the region dominated by the stellar wind., Comment: Published in Proceedings of High Energy Phenomena in Relativistic Outflows II, held in Buenos Aires, 26-30 October 2009, ed. G. Romero

The stable propagation of jets in FRII sources is remarkable if one takes into account that large-scale jets are subjected to potentially highly disruptive three-dimensional (3D) Kelvin-Helmholtz instabilities. Numerical simulations can address this problem and help clarify the causes of this remarkable stability. Following previous studies of the stability of relativistic flows in two dimensions (2D), it is our aim to test and extend the conclusions of such works to three dimensions. We present numerical simulations for the study of the stability properties of 3D, sheared, relativistic flows. This work uses a fully parallelized code Ratpenat that solves equations of relativistic hydrodynamics in 3D. The results of the present simulations confirm those in 2D. We conclude that the growth of resonant modes in sheared relativistic flows could be important in explaining the long-term collimation of extragalactic jets., Comment: Accepted for publication in A&A

High-mass microquasars may produce jets that will strongly interact with surrounding stellar winds on binary system spatial scales. We study the dynamics of the collision between a mildly relativistic hydrodynamical jet of supersonic nature and the wind of an OB star. We performed numerical 3D simulations of jets that cross the stellar wind with the code Ratpenat. The jet head generates a strong shock in the wind, and strong recollimation shocks occur due to the initial overpressure of the jet with its environment. These shocks can accelerate particles up to TeV energies and produce gamma-rays. The recollimation shock also strengthens jet asymmetric Kelvin-Helmholtz instabilities produced in the wind/jet contact discontinuity. This can lead to jet disruption even for jet powers of several times $10^{36}$ erg s$^{-1}$. High-mass microquasar jets likely suffer a strong recollimation shock that can be a site of particle acceleration up to very high energies, but also eventually lead to the disruption of the jet., Comment: Accepted for publication in A&A Letters

The high kinetic energy outflowing in the jets of microquasars is delivered to the surrounding interstellar medium. This energy input can cause the formation of bow shocks and cocoons that may be detectable from radio to gamma-ray energies. Evidences or hints of emission from jet/medium interactions have been reported for some sources, but little has been done regarding the theoretical modeling of the resulting non-thermal emission. We have developed an analytical model based on those successfully applied to extragalactic sources or the interaction of AGN jets with their surroundings. Focusing on the adiabatic phase of the growing structures, we give estimations of the expected luminosities through synchrotron, relativistic Bremsstrahlung and inverse Compton processes. We conclude that the interaction structures may be detectable at radio wavelengths, while extreme values for the jet kinetic power, the source age and the medium density are required to make the emission at high and very high energies detectable., Comment: Contribuition to the 7th. Microquasar Workshop, Foca 2008

The interaction of microquasar jets with their environment can produce non-thermal radiation as is the case for extragalactic outflows impacting on their surroundings. We have developed an analytical model based on those successfully applied to extragalactic sources. The jet is taken to be a supersonic and mildly relativistic hydrodynamical outflow. We focus on the jet/shocked medium structure when being in its adiabatic phase, and assume that it grows in a self-similar way. We calculate the fluxes and spectra of the radiation produced via synchrotron, Inverse Compton and relativistic Bremsstrahlung processes by electrons accelerated in strong shocks. A hydrodynamical simulation is also performed to further investigate the jet interaction with the environment and check the physical parameters used in the analytical model. We conclude that microquasar jet termination regions could be detectable at radio wavelengths for current instruments sensitive to arcminute scales while at X-rays the expected luminosities are moderate, although the emitter is more compact than the radio one. The radiation at gamma-ray energies may be within the detection limits of the next generation of satellite and ground-based instruments., Comment: Accepted for publication in Astronomy & Astrophysics