Superluminal Knots Research Articles

Polarimetric Observations of 15 Active Galactic Nuclei at High Frequencies: Jet Kinematics from Bimonthly Monitoring with the Very Long Baseline Array

We present total and polarized intensity images of 15 active galactic nuclei obtained with the Very Long Baseline Array at 7 mm wavelength at 17 epochs from 1998 March to 2001 April. At some epochs the images are accompanied by nearly simultaneous polarization measurements at 3 mm, 1.35/0.85 mm, and optical wavelengths. Here we analyze the 7 mm images to define the properties of the jets of two radio galaxies, five BL Lac objects, and eight quasars on angular scales 0.1 mas. We determine the apparent velocities of 106 features in the jets. For many of the features we derive Doppler factors using a new method based on a comparison of the timescale of decline in flux density with the light-travel time across the emitting region. This allows us to estimate the Lorentz factors (Γ), intrinsic brightness temperatures, and viewing angles of 73 superluminal knots, as well as the opening angle of the jet for each source. The Lorentz factors of the jet flows in the different blazars range from Γ ~ 5 to 40 with the majority of the quasar components having Γ ~ 16–18, while the values in the BL Lac objects are more uniformly distributed. The brightest knots in the quasars have the highest apparent speeds, while the more slowly moving components are pronounced in the BL Lac objects. The quasars in our sample have similar opening angles and marginally smaller viewing angles than the BL Lacs. The two radio galaxies have lower Lorentz factors and wider viewing angles than the blazars. Opening angle and Lorentz factor are inversely proportional, as predicted by gasdynamical models. The brightness temperature drops more abruptly with distance from the core in the BL Lac objects than in the quasars and radio galaxies, perhaps owing to stronger magnetic fields in the former resulting in more severe synchrotron losses of the highest energy electrons. In nine sources we detect statistically meaningful deviations from ballistic motion, with the majority of components accelerating with distance from the core. In six sources we identify jet features with characteristics of trailing shocks that form behind the primary strong perturbations in jet simulations. The apparent speeds of these components increase with distance from the core, suggestive of acceleration of the underlying jet.

Do the Mildly Superluminal VLBI Knots Exclude Ultrarelativistic Blazar Jets?

We compute the effective values of apparent transverse velocity and flux boosting factors for the VLBI radio knots of blazar jets by integrating over the angular distributions of these quantities across the widths of jets with finite opening angles but constant velocities. For high bulk Lorentz factors (Γ > 10), variations across the jet can be quite large if the opening angle, ω, is even a few degrees on subparsec scales. The resulting apparent speeds are often much lower than those obtained from the usual analyses that ignore the finite jet opening angles. We can thus reconcile the usually observed subluminal or mildly superluminal speeds with the very high (20) Γ factors, required by the inverse Compton origin and rapid variability of TeV fluxes, as well as by intraday radio variability. Thus it is possible to associate the VLBI radio knots directly with shocks in the ultrarelativistic main jet flow, without invoking very rapid jet deceleration on parsec scales or extremely unlikely viewing angles.

Observational evidence for the accretion-disk origin for a radio jet in an active galaxy.

Accretion of gas onto black holes is thought to power the relativistic jets of material ejected from active galactic nuclei (AGN) and the 'microquasars' located in our Galaxy. In microquasars, superluminal radio-emitting features appear and propagate along the jet shortly after sudden decreases in the X-ray fluxes. This establishes a direct observational link between the black hole and the jet: the X-ray dip is probably caused by the disappearance of a section of the inner accretion disk as it falls past the event horizon, while the remainder of the disk section is ejected into the jet, creating the appearance of a superluminal bright spot. No such connection has hitherto been established for AGN, because of insufficient multi-frequency data. Here we report the results of three years of monitoring the X-ray and radio emission of the galaxy 3C120. As has been observed for microquasars, we find that dips in the X-ray emission are followed by ejections of bright superluminal knots in the radio jet. The mean time between X-ray dips appears to scale roughly with the mass of the black hole, although there are at present only a few data points.

Monthly 43 GH[CLC]z[/CLC] VLBA Polarimetric Monitoring of 3C 120 over 16 Epochs: Evidence for Trailing Shocks in a Relativistic Jet

We present a 16 month sequence of monthly polarimetric 43 GHz VLBA images of the radio galaxy 3C 120. The images probe the inner regions of the radio jet of this relatively nearby superluminal radio galaxy at a linear resolution of 0.07 h pc (H0 = 65 h65 km s-1 Mpc-1). We follow the motion of a number of features with apparent velocities between 4.01 ± 0.08 and 5.82 ± 0.13 h c. A new superluminal knot, moving at 4.29 ± 0.16 h c, is observed to be ejected from the core at a time coincident with the largest flare ever observed for this source at millimeter wavelengths. Changes in the position angle of this component, as well as a progressive rotation of its magnetic polarization vector, suggest the presence of a twisted (resembling a helix in projection) configuration of the underlying jet magnetic field and jet geometry. We identify several knots that appear in the wake of the new superluminal component, moving at proper motions ~4 times slower than any of the other moving knots observed in 3C 120. These features have properties similar to those of the shocks seen in relativistic, time-dependent, hydrodynamical, and emission simulations of compact jets. Such trailing compressions are triggered by pinch-mode jet-body instabilities caused by the propagation of a strong perturbation, which we associate with the new strong superluminal component.

Spectral Reversal and Stratification of the Jet in 3C 273

Quasi-simultaneous VLBI observations at 15-86GHz have shown that in the classical superluminal radio source 3C273, the spectral index (S / ) has a systematic variation along the jet. For epoch 1995.15, a spectral reversal was observed at core distance 1.5mas, where the superluminal knot C12 located. Similarly, for epoch 1997.18, two spectral reversals were observed at core distances of 1.8mas and 4.2mas, where superluminal knots C11 and C14 were, respectively. These spectral reversals are associated with local maxima of the jet width. We suggest that this phenomenon may be related to a stratification of the jet structure, i.e., its physical parameters (flow velocity, Doppler factor, electron density and energy, magnetic field strength, etc.) are substantially dependent on the distance from the jet axis. These properties may be naturally formed through gasdynamic processes when the jet expands into a lower pressure ambient medium.

Multiepoch Very Long Baseline Array Observations of EGRET‐detected Quasars and BL Lacertae Objects: Connection between Superluminal Ejections and Gamma‐Ray Flares in Blazars

We examine the coincidence of times of high γ-ray flux and ejections of superluminal components from the core in EGRET blazars based on a Very Long Baseline Array (VLBA) monitoring program at 22 and 43 GHz from 1993 November to 1997 July. In 23 cases of γ-ray flares for which sufficient VLBA data exist, 10 of the flares (in eight objects) fall within 1 σ uncertainties of the extrapolated epoch of zero separation from the core of a superluminal radio component. In each of two sources (0528+134 and 1730-130), two successive γ-ray flares were followed by the appearance of new superluminal components. We carried out statistical simulations that show that if the number of coincidences is ≥10, the radio and γ-ray events are associated with each other at greater than 99.999% confidence. Our analysis of the observed behavior, including variability of the polarized radio flux, of the sources before, during, and after the γ-ray flares suggests that the γ-ray events occur in the superluminal radio knots. This implies that the γ-ray flares are caused by inverse Compton scattering by relativistic electrons in the parsec-scale regions of the jet rather than closer to the central engine.

Periodic Variations of the Jet Flow Lorentz Factor in 3C 273

3C 273 has been observed with VLBI for more than 30 years. The entire data have shown that the position angle of the superluminal knots ejected from the core varies periodically with a period of ~15 years. Moreover, their apparent velocity observed during the period 1963 – 1997 has systematically decreased by a ≳ 2. These remarkable properties are explained in terms of a precessing factor of ~ jet model, in which the ejection Lorentz factor of the superluminal knots has been decreasing during the last thirty years and has superposed on it a short-term (~5 year) oscillation. The periodic variations derived by the model-fitting are compared with the variations in the optical flux density. Binary black hole models are briefly discussed to show possible relations of the observed periods to the periods involved in a binary system (orbital motion, spin of black hole, accretion-disk rotation and Newtonian-driven precession etc.).

Internal shocks in the jets of radio-loud quasars

The central engine causing the production of jets in radio sources may work intermittently, accelerating shells of plasma with different mass, energy and velocity. Faster but later shells can then catch up slower earlier ones. In the resulting collisions shocks develop, converting some of the ordered bulk kinetic energy into magnetic field and random energy of the electrons which then radiate. We propose that this internal shock scenario, which is the scenario generally thought to explain the observed gamma-ray burst radiation, can also work for radio sources in general, and for blazars in particular. We investigate in detail this idea, simulating the birth, propagation and collision of shells, calculating the spectrum produced in each collision, and summing the locally produced spectra from those regions of the jet which are simultaneously active in the observer's frame. We can thus construct snapshots of the overall spectral energy distribution, time-dependent spectra and light curves. This allows us to characterize the predicted variability at any frequency, study correlations between the emission at different frequencies, specify the contribution of each region of the jet to the total emission, and find correlations between flares at high energies and the birth of superluminal radio knots and/or radio flares. The model has been applied to reproduce qualitatively the observed properties of 3C 279. Global agreement in terms of both spectra and temporal evolution is found. In a forthcoming work, we will explore the constraints that this scenario sets on the initial conditions of the plasma injected in the jet and the shock dissipation for different classes of blazars.

Simulations of superluminal radio sources

The superluminal knots of VLBI-sources are widely believed to be shocks travelling along relativistic jets. So far only very simple quasi-one dimensional models have been used to study this phenomenon. Recent progress in numerical methods for relativistic fluid dynamics (Falle & Komissarov 1995) make it possible to carry out much more realistic simulations of such flows.

The spectral evolution of superluminal radio knots: A shock-in-jet model with prolonged energy supply

In this paper a jet-shock model is proposed which includes a continuous supply of energy and adiabatic expansion of the plasma behind the shock. Our model calculations reproduce the three-stage evolution of the turnover flux-frequency curve, observed in the superluminal knot C 4 of 3C,345. By considering a high-energy cutoff in the input electron spectrum, the steepening of the spectrum for ν ≳ 20 GHz, observed in C 4, is also reproduced.

Simulations of Superluminal Radio Sources

The superluminal knots of VLBI-sources are widely believed to be shocks travelling along relativistic jets. So far only very simple quasi-one dimensional models have been used to study this phenomenon. Recent progress in numerical methods for relativistic fluid dynamics (Falle & Komissarov 1995) make it possible to carry out much more realistic simulations of such flows.

Long-term variability of 3C 273 at 22 and 44 GHz

Observations of 3C 273 at 22 and 44 GHz during the past 7 yr are presented. Between 1981 and 1984, the QSO was in a high-flux-density state at both frequencies, the intensity decreasing by a factor of 2 during 1985, reaching a minimum value in April 1986. The changes in the flux density were larger at 44 GHz and preceded the changes at 22 GHz. The variability in 1983 was identified with the millimeter and infrared outburst of February 1983, and the observed flux-density fluctuations were correlated with the appearance of the superluminal knots observed at 10.7 GHz with VLBI techniques. 46 references.

Investigations of 3C345

The well known superluminal source 3C345 is associated with a 16th magnitude quasar at z=0.595. The optical continuum is highly polarized and shows violent variations. At x-ray frequencies it is a weak source; while at radio frequencies it is bright and variable. The radio structure consists of a 200 kpc halo, a 20 kpc jet, and a very bright pc scale region which shows superluminal motion. Recent MkII VLBI monitoring (Biretta, Moore, Cohen 1986) at 2, 5, 11, and 22 GHz has provided accurate spectra, sizes, expansion rates, velocities, and trajectories for the superluminal knots and the “core.” Here we consider physical conditions in the emission regions and models for the kinematics.

Magnetic-field configurations in compact extragalactic jets

Multifrequency and multitime polarimetry of active, compact radio sources strongly suggest that the magnetic-field structures in these sources are largely disordered or turbulent. Some initial, detailed model calculations of relativistic jets with turbulent fields have been performed in an effort to better understand the relationships between the observed polarization properties and physical structures of sources. Simulations are described involving nearly steady, isothermal, constant velocity jets with magnetic fields having approximately a Kolmogorov turbulence spectrum that is carried along the jet. They produce structures similar to those actually observed, including a core-jet appearance with superluminal knots. Likewise, the polarization has a behavior in frequency and time that is at least qualitatively similar to that observed. In addition, although individual portions of the model sources are highly polarized, as also seen with very long baseline interferometry, the integrated polarizations are relatively small, as observed.