Very long baseline interferometry (VLBI) imaging of radio emission from extragalactic jets provides a unique probe of physical mechanisms governing the launching, acceleration, and collimation of relativistic outflows. The two-dimensional structure and kinematics of the jet in M\,87 (NGC\,4486) have been studied by applying the Wavelet-based Image Segmentation and Evaluation (WISE) method to 11 images obtained from multi-epoch Very Long Baseline Array (VLBA) observations made in January-August 2007 at 43 GHz ($\lambda = 7$ mm). The WISE analysis recovers a detailed two-dimensional velocity field in the jet in M\,87 at sub-parsec scales. The observed evolution of the flow velocity with distance from the jet base can be explained in the framework of MHD jet acceleration and Poynting flux conversion. A linear acceleration regime is observed up to $z_{obs} \sim 2$\,mas. The acceleration is reduced at larger scales, which is consistent with saturation of Poynting flux conversion. Stacked cross correlation analysis of the images reveals a pronounced stratification of the flow. The flow consists of a slow, mildly relativistic layer (moving at $\beta \sim 0.5\,c$), associated either with instability pattern speed or an outer wind, and a fast, accelerating stream line (with $\beta \sim 0.92$, corresponding to a bulk Lorentz factor $\gamma \sim 2.5$). A systematic difference of the apparent speeds in the northern and southern limbs of the jet is detected, providing evidence for jet rotation. The angular velocity of the magnetic field line associated with this rotation suggests that the jet in M87 is launched in the inner part of the disk, at a distance $r_0 \sim 5\, R_\mathrm{s}$ from the central engine. The combined results of the analysis imply that MHD acceleration and conversion of Poynting flux to kinetic energy play the dominant roles in collimation and acceleration of the flow in M\,87.

One of the key open questions in the study of relativistic jets is their interaction with the environment. Here, we study the initial evolution of both electron-proton and electron-positron relativistic jets, focusing on their lateral interaction with the ambient plasma. We trace the generation and evolution of the toroidal magnetic field generated by both kinetic Kelvin-Helmholtz (kKH) and Mushroom instabilities (MI). This magnetic field collimates the jet. We show that in electron-proton jet, electrons are perpendicularly accelerated with jet collimation. The magnetic polarity switches from the clockwise to anti-clockwise in the middle of jet, as the instabilities weaken. For the electron-positron jet, we find strong mixture of electron-positron with the ambient plasma, that results in the creation of a bow shock. Merger of magnetic field current filaments generate density bumps which initiate a forward shock. The strong mixing between jet and ambient particles prevents full development of the jet on the studied scale. Our results therefore provide a direct evidence for both jet collimation and particle acceleration in the created bow shock. Differences in the magnetic field structures generated by electron-proton and electron-positron jets may contribute to observable differences in the polarized properties of emission by electrons.

Abstract. We have investigated the generation of magnetic fields associated with velocity shear between an unmagnetized relativistic jet and an unmagnetized sheath plasma. We have examined the strong magnetic fields generated by kinetic shear (Kelvin–Helmholtz) instabilities. Compared to the previous studies using counter-streaming performed by Alves et al. (2012), the structure of the kinetic Kelvin–Helmholtz instability (KKHI) of our jet-sheath configuration is slightly different, even for the global evolution of the strong transverse magnetic field. In our simulations the major components of growing modes are the electric field Ez, perpendicular to the flow boundary, and the magnetic field By, transverse to the flow direction. After the By component is excited, an induced electric field Ex, parallel to the flow direction, becomes significant. However, other field components remain small. We find that the structure and growth rate of KKHI with mass ratios mi/me = 1836 and mi/me = 20 are similar. In our simulations saturation in the nonlinear stage is not as clear as in counter-streaming cases. The growth rate for a mildly-relativistic jet case (γj = 1.5) is larger than for a relativistic jet case (γj = 15).

Jets in low-luminosity radio galaxies are known to decelerate from relativistic speeds on parsec scales to mildly or sub-relativistic speeds on kiloparsec scales. Several mechanisms have been proposed to explain this effect, including strong reconfinement shocks and the growth of instabilities (both leading to boundary-layer entrainment) and mass loading from stellar winds or molecular clouds. We have performed a series of axisymmetric simulations of the early evolution of jets in a realistic ambient medium to probe the effects of mass loading from stellar winds using the code Ratpenat. We study the evolution of Fanaroff-Riley Class I (FRI) jets, with kinetic powers L_j \sim 1.e41-1.e44 erg/s, within the first 2 kpc of their evolution, where deceleration by stellar mass loading should be most effective. Mass entrainment rates consistent with present models of stellar mass loss in elliptical galaxies produce deceleration and effective decollimation of weak FRI jets within the first kiloparsec. However, powerful FRI jets are not decelerated significantly. In those cases where the mass loading is important, the jets show larger opening angles and decollimate at smaller distances, but the overall structure and dynamics of the bow-shock are similar to those of unloaded jets with the same power and thrust. According to our results, the flaring observed on kpc scales is initiated by mass loading in the weaker FRI jets and by reconfinement shocks or the growth of instabilities in the more powerful jets. The final mechanism of decollimation and deceleration is always the development of disruptive pinching modes.

Helical structures are common in extragalactic jets. They are usually attributed in the literature to periodical phenomena in the source (e.g., precession). In this work, we use very long baseline interferometry data of the radio jet in the quasar S5 0836+710 and hypothesize that the ridgeline of helical jets like this corresponds to a pressure maximum in the jet and assume that the helically twisted pressure maximum is the result of a helical wave pattern. For our study, we use observations of the jet in S5 0836+710 at different frequencies and epochs. The results show that the structures observed are physical and not generated artificially by the observing arrays. Our hypothesis that the observed intensity ridgeline can correspond to a helically twisted pressure maximum is confirmed by our observational tests. This interpretation allows us to explain jet misalignment between parsec and kiloparsec scales when the viewing angle is small, and also brings us to the conclusion that high-frequency observations may show only a small region of the jet flow concentrated around the maximum pressure ridgeline observed at low frequencies. Our work provides a potential explanation for the apparent transversal superluminal speeds observed in several extragalactic jets by means of transversal shift of an apparent core position with time.

Context. The remarkable stability of extragalactic jets is surprising, given the reasonable possibility of the growth of instabilities. In addition, much work in the literature has invoked this possibility to explain observed jet structures and obtain information about the jet from these structures. For example, it has been shown that the observed helical structures in the jet in S5 0836+710 could be associated with helical pressure waves generated by a Kelvin-Helmholtz instability.Aims. Our aim is to resolve the arc-second structure of the jet in the quasar S5 0836+710 and confirm the lack of a hot-spot (reverse jet-shock) found by present observing arrays, as this lack implies a loss of jet collimation before interaction with the intergalactic medium.Methods. We use an observation performed in 2008 using EVN (European Very Long Baseline Interferometry Network) and MERLIN (Multi-Element Radio Linked Interferometer Network). The resultant combined image after data reduction has provided a complete image of the object on arc-second scales.Results. The lack of a hot-spot in the arc-second radio structure is taken as evidence that the jet loses its collimation between the VLBI (Very Long Baseline Interferometry) region and the region of interaction with the ambient medium.Conclusions. This result and the previous identification of the helical structures in the jet with helical pressure waves that grow in amplitude with distance allow us to conclude that the jet is probably disrupted by the growth of Kelvin-Helmholtz instability. This observational evidence confirms that the physical parameters of jets can be extracted using the assumption that instability is present in jets and can be the reason for many observed structures. Interestingly, the observed jet is classified as a FRII (Fanaroff-Riley type II) object in terms of its luminosity, but its large-scale morphology does not correspond to this classification. The implications of this finding are discussed.

Radio and optical images of the M 87 jet show bright filaments, twisted into an apparent double helix, extending from HST-1 to knot A. Proper motions within the jet suggest a decelerating jet flow passing through a slower, accelerating wave pattern. We use these observations to develop a mass and energy flux-conserving model describing the jet flow and conditions along the jet. Our model requires the jet to be an internally hot, but subrelativistic plasma, from HST-1 to knot A. Subsequently, we assume that the jet is in pressure balance with an external cocoon and we determine the cocoon conditions required if the twisted filaments are the result of the Kelvin–Helmholtz (KH) unstable elliptical mode. We find that the cocoon must be cooler than the jet at HST-1 but must be about as hot as the jet at knot A. Under these conditions, we find that the observed filament wavelength is near the elliptical mode maximum growth rate and growth is rapid enough for the filaments to develop and saturate well before HST-1. We generate a pseudo-synchrotron image of a model jet carrying a combination of normal modes of the KH instability. The pseudo-synchrotron image of the jet reveals that (1) a slow decline in the model jet's surface brightness is still about five times faster than the real jet, (2) KH-produced dual helically twisted filaments can appear qualitatively similar to those on the real jet if any helical perturbation to the jet is very small or nonexistent inside knot A, and (3) the knots in the real jet cannot be associated with the twisted filamentary features and are unlikely to be the result of a KH instability. The existence of the knots in the real jet, the limb brightening of the real jet in the radio, and the slower decline of the surface brightness of the real jet indicate that additional processes—such as unsteady jet flow and internal particle acceleration—are occurring within the jet. Disruption of the real jet beyond knot A by KH instability is consistent with the jet and cocoon conditions we find at knot A.

We investigated the stability of magnetized relativistic jets through 3D relativistic MHD simulations. First we investigated the stability of magnetized spine-sheath relativistic jets against Kelvin-Helmholtz (KH) instability. We found that destructive KH modes can be stabilized even when the jet; Lorentz factor significantly exceeds the Alfven Lorentz factor. Second we studied the development of current-driven (CD) kink instability of a static force-free helical magnetic field configuration. We found that the initial configuration is strongly distorted but not disrupted by CD kink instability. The linear growth and nonlinear evolution of the CD kink instability depends on the radial density profile and strongly depends on the magnetic pitch profile.

We solve the Riemann problem for the deceleration of an arbitrarily magnetized relativistic flow injected into a static unmagnetized medium in one dimension. We find that for the same initial Lorentz factor, the reverse shock becomes progressively weaker with increasing magnetization σ(the Poynting-to kinetic energy flux ratio), and the shock becomes a rarefaction wave when σexceeds a critical value, σ_c, defined by the balance between the magnetic pressure in the flow and the thermal pressure in the forward shock. In the rarefaction wave regime, we find that the rarefied region is accelerated to a Lorentz factor that is significantly larger than the initial value. This acceleration mechanism is due to the strong magnetic pressure in the flow. We discuss the implications of these results for models of gamma-ray bursts and active galactic nuclei.

The accretion of matter onto a massive black hole is believed to feed the relativistic plasma jets found in many active galactic nuclei (AGN). Although some AGN accelerate particles to energies exceeding 10(12) electron volts and are bright sources of very-high-energy (VHE) gamma-ray emission, it is not yet known where the VHE emission originates. Here we report on radio and VHE observations of the radio galaxy Messier 87, revealing a period of extremely strong VHE gamma-ray flares accompanied by a strong increase of the radio flux from its nucleus. These results imply that charged particles are accelerated to very high energies in the immediate vicinity of the black hole.

Plasma instabilities (e.g., Buneman, Weibel and other two-stream instabilities) excited in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a new 3-D relativistic particle-in-cell code, we have investigated the particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an unmagnetized electron-positron plasma. The simulation has been performed using a long simulation system in order to study the nonlinear stages of the Weibel instability, the particle acceleration mechanism, and the shock structure. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic (HD) like shock structure. In the leading shock, electron density increases by a factor of 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. We discuss the possible implication of our simulation results within the AGN and GRB context.

With its high black hole mass, proximity, and bright jet, M87 provides the best prospect for a direct imaging study of the acceleration and collimation region of a jet. Previous VLBI observations have shown an edge brightened structure with a wide opening angle at small scales. An effort to measure component speeds in this region using existing VLBA data at 43 GHz gave tentative results of 0.25c to 0.4c but also indicated that faster sampling is needed. Here we provide a progress report on a project to make a properly sampled movie of motions in the inner jet using the VLBA at 43 GHz. A pilot project during 2006 measured speeds of about 0.6c and was used to set a frame interval of 3 weeks for the movie. The movie observations began in January 2007. Results from the pilot and from the first frame of the movie are presented. The goal of the project is to provide observations of the structure and dynamics of the jet on scales from under 100 to a few hundred Schwarzschild radii that can be compared with expectations from theoretical studies and numerical modeling.

We have developed a new three-dimensional general relativistic magnetohydrodynamic code by using a conservative, high-resolution shock-capturing scheme. The numerical fluxes are calculated using the HLL approximate Riemann solver scheme. The flux-CT scheme is used to maintain a divergence-free magnetic field. Various 1-dimensional test problems show significant improvements over our previous GRMHD code. We have performed simulations of jet formations from a geometrically thin accretion disk near a non-rotating and a rotating black hole. The new simulation results show that the jet is formed by the same manner as in previous works and propagates outward. As the magnetic field strength becomes weaker, larger amount of matter launches with the jet. On the other hand when the magnetic field strength becomes stronger, the jet has less-matter and becomes poynting flux dominated. We will also discuss how the jet properties depend on the rotation of a black hole.

Plasma instabilities (e.g., Buneman, Weibel, and other two-stream instabilities) created in collisionless shocks may be responsible for particle (electron, positron, and ion) acceleration. Using a three-dimensional relativistic electromagnetic particle code, we have investigated long-term particle acceleration associated with relativistic electron-ion or electron-positron jet fronts propagating into an unmagnetized ambient electron-ion or electron-positron plasma. These simulations have been performed with a longer simulation system than our previous simulations in order to investigate the nonlinear stage of the Weibel instability and its particle acceleration mechanism. The current channels generated by the Weibel instability are surrounded by toroidal magnetic fields and radial electric fields. This radial electric field is quasi stationary and accelerates particles that are then deflected by the magnetic field. Whether particles are accelerated or decelerated along the jet propagation direction depends on the velocity of particles and the sign of × in the moving frame of each particle. For the electron-ion case the large-scale current channels generated by the ion Weibel instability lead to more acceleration near the jet head. Consequently, the accelerated jet electrons in the electron-ion jet have a significant hump above a thermal distribution. However, in the electron-positron case, accelerated jet electrons have a smoother, nearly thermal distribution. In the electron-positron case, initial acceleration occurs as current channels form and then continues at a much lesser rate as the current channels and corresponding toroidal magnetic fields generated by the Weibel instability dissipate.

We have applied numerical simulations and modeling to the particle acceleration, magnetic field generation, and emission from relativistic shocks. We investigate the nonlinear stage of theWeibel instability and compare our simulations with the observed gamma-ray burst emission. In collisionless shocks, plasma waves and their associated instabilities (e.g., the Weibel, Buneman and other two-stream instabilities) are responsible for particle (electron, positron, and ion) acceleration and magnetic field generation. 3-D relativistic electromagnetic particle (REMP) simulations with three different electron-positron jet velocity distributions and also with an electron-ion plasma have been performed and show shock processes including spatial and temporal evolution of shocks in unmagnetized ambient plasmas. The growth time and nonlinear saturation levels depend on the initial jet parallel velocity distributions. Simulations show that the Weibel instability created in the collisionless shocks accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The nonlinear fluctuation amplitude of densities, currents, electric, and magnetic fields in the electron-positron shocks are larger for smaller jet Lorentz factor. This comes from the fact that the growth time of the Weibel instability is proportional to the square of the jet Lorentz factor. We have performed simulations with broad Lorentz factor distribution of jet electrons and positrons, which is assumed to be created by photon annihilation. Simulation results with this broad distribution show that the Weibel instability is excited continuously by the wide-range of jet Lorentz factor from lower to higher values. In all simulations the Weibel instability is responsible for generating and amplifying magnetic fields perpendicular to the jet propagation direction, and contributes to the electron's (positron's) transverse deflection behind the jet head. This small scale magnetic field structure contributes to the generation of "jitter" radiation from deflected electrons (positrons), which is different from synchrotron radiation in uniform magnetic fields. The jitter radiation resulting from small scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks. The detailed studies of shock microscopic process evolution may provide some insights into early and later GRB afterglows.

Context. Relativistic outflows represent one of the best-suited tools to probe the physics of AGN. Numerical modelling of internal structure of the relativistic outflows on parsec scales provides important clues about the conditions and dynamics of the material in the immediate vicinity of the central black holes in AGN. Aims. We investigate possible causes of the structural patterns and regularities observed in the parsec-scale jet of the well-known quasar 3C 273. Methods. We present here the results from a 3D relativistic hydrodynamics numerical simulation based on the parameters given for the jet by Lobanov & Zensus (2001, Science, 294, 128), and one in which the effects of jet precession and the injection of discrete components have been taken into account. We compare the model with the structures observed in 3C 273 using very long baseline interferometry and constrain the basic properties of the flow. Results. We find growing perturbation modes in the simulation with similar wavelengths to those observed, but with a different set of wave speeds and mode identification. If the observed longest helical structure is produced by the precession of the flow, longer precession periods should be expected. Conclusions. Our results show that some of the observed structures could be explained by growing Kelvin-Helmholtz instabilities in a slow moving region of the jet. However, we point towards possible errors in the mode identification that show the need of more complete linear analysis in order to interpret the observations. We conclude that, with the given viewing angle, superluminal components and jet precession cannot explain the observed structures.

Many jets exhibit twisted helical structures. Where superluminal motions are detected, jet orientation and pattern/flow speed are considerably constrained. In this case modeling efforts can place strong limits on conditions in the jet and in the external environment. This can be done by modeling the spatial development of helical structures that are sensitively dependent on these conditions. Along an expanding jet, this sensitivity manifests itself in predictable changes in pattern speed and observed wavelength. In general, twists of low frequency relative to the local resonant frequency are advected along the expanding jet into a region in which the twist frequency is high relative to the local resonant frequency. The wave speed can be very different in these two frequency regimes. Potential effects include helical twists with a nearly constant apparent wavelength, an apparent wavelength scaling approximately with the jet radius for up to 2 orders of magnitude of jet expansion, or multiple twist wavelengths with vastly different intrinsic scale and vastly different wave speeds that give rise to similar observed twist wavelengths but with very different observed motion. In this paper I illustrate the basic intrinsic and observed behavior of these structures and show how to place constraints on jet conditions in superluminal jets using the apparent structures and motions in the inner 3C 120 jet.

In this paper we use the predicted spatial development of helical structures along an expanding jet to model observed structures and motions in the 3C 120 jet. New results of VLBI imaging of the parsec-scale radio jet in 3C 120 at 5 GHz are examined along with older long-term monitoring results at 5 GHz and older results obtained at 22 and 43 GHz. The high-frequency observations provide detailed information on motions and structure from 0.5 to 10 mas from the core and the lower frequency observations from 1 to 30 mas from the core. Proper motions of helical components associated with the pattern and of other components that move through the pattern provide estimates of flow and helical pattern speeds. Theoretical modeling of the motion and appearance of the helical pattern allows determination of sound speeds as a function of the jet viewing angle. The jet sound speed declines although probably not as fast as adiabatically. At a 12degrees viewing angle the most likely scenario involves a decline in jet sound speed from c/3 < a(j) < c/root3 at similar to 0.5 mas from the core to 0.1c < a(j) < 0.25c at similar to 25 mas from the core accompanied by some acceleration in the jet flow from Lorentz factor gamma less than or similar to 5 to gamma greater than or similar to 7. The sound speed in the cocoon medium around the jet is less well determined but is less than the sound speed in the jet probably by a factor of 1.5 - 5. A largest possible viewing angle of 15degrees implies a jet sound speed at the upper limit of these estimates and somewhat higher flow Lorentz factors. However, jet morphology argues against viewing angles larger than 12degrees. At smaller viewing angles the jet sound speed is lower, and at a 6degrees viewing angle the jet sound speed is about a factor of 2 less, but the flow Lorentz factor is comparable. The decline in radio intensity is on the order of what would be associated with isothermal jet expansion. Knot-interknot intensity variations are greater than would be expected from adiabatic compressions associated with the helical twist, and we infer the presence of a shock along the leading edge of the helical twist in addition to shock or density structures flowing through the helical pattern. Our results imply that the macroscopic heating of the expanding jet fluid is less than the microscopic energization of the synchrotron radiating relativistic electrons.