Superluminal Components Research Articles

The role of Kelvin-Helmholtz instability in the internal structure of relativistic outflows. The case of the jet in 3C 273

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.

The blazar 0059+581: Successful prognosis of activity

Our earlier joint analysis of light curves for the blazar 0059+581 at 4.8, 8, 14.5, 22, and 37 GHz with high-resolution VLBI images led us to suggest that the activity in this source develops in cycles, or periods, with a duration of about four years, with a “typical scenario” for the development of the source’s activity taking place over a cycle. Based on this analysis, we predicted in 2002 that a new superluminal component would be ejected from the core of this source in a structural position angle ∼170° no later than by the end of 2003. A 43-GHz VLBI image obtained on September 14, 2003, as part of a program to monitor the structure of reference sources used for a radio astronomical coordinate system, convincingly confirms the correctness of this prediction. This is the first time in the history of radio astronomy that a new superluminal component has been detected at a predicted time and in a predicted structural position angle.

The intra-night optical variability of the bright BL Lacertae object S5 0716+714

Aims.We address the topic of the intra-night optical variability of the BL Lac object S5 0716+714.

Can long-term periodic variability and jet helicity in 3C 120 be explained by jet precession?

Optical variability of 3C 120 is discussed in the framework of jet precession. Specifically, we assume that the observed long-term periodic variability is produced by the emission from an underlying jet with a time-dependent boosting factor driven by precession. The differences in the apparent velocities of the different superluminal components in the milliarcsec jet can also be explained by the precession model as being related to changes in the viewing angle. The evolution of the jet components has been used to determine the parameters of the precession model, which also reproduce the helical structure seen at large scales. Among the possible mechanisms that could produce jet precession, we consider that 3C 120 harbours a supermassive black hole binary system in its nuclear region, and that torques induced by misalignment between the accretion disc and the orbital plane of the secondary black hole are responsible for this precession; we estimate upper and lower limits for the black holes masses and their mean separation.

Jet precession and its observational evidence: The cases of 3C 345 and 3C 120

Several radio-loud objects exhibit a complex structure when observed at radio wavelengths: a stationary core, which is thought to harbour the central engine that powers the AGN phenomena, and a relativistic jet, formed by several superluminal components. In some cases, jet components are ejected with different apparent proper motions and directions on the plane of the sky. Moreover, these sources can also show signatures of long-term periodic variability in their historical optical light curve. In this work, we selected the objects 3C 120 and 3C 345, which exhibit both characteristics mentioned above, and interpret them in the framework of jet inlet precession. A brief discussion about what kind of mechanism could be responsible for jet precession is also presented.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Intensive VLBI monitoring programs vs. numerical simulations: the jet in 3C 120

Comparison between intensive VLBI monitoring programs and relativistic time-dependent hydrodynamical and emission simulations has proven the value of such computations for interpreting observations of actual sources. The observed inner jet emission structure is shown to be greatly determined by the non-linear hydrodynamical evolution of traveling perturbations interacting with the underlying jet and external medium. Monthly observations of the radio galaxy 3C 120 spanning for more than two years have revealed that multiple jet features (trailing components) can be observed to emerge from the hydrodynamical evolution of superluminal components along the jet.

Optical monitoring of the BL Lac object ON 231 in 1972–1990

The results of 19-year optical monitoring of ON 231 are presented. Photographic B-band observations were obtained during 123 nights from April 1972 to March 1990 as part of the quasar monitoring program carried out the Astronomical Institute of St. Petersburg State University. The new combined B-band light curve for ON 231 (1892–1999) is appreciably improved by our data, enabling us to identify a number of previously undetected high-amplitude events with time scales of weeks to months. On two nights, intraday variability with amplitudes \( \sim 0\mathop .\limits^m 8 - 0\mathop .\limits^m 9\) was observed. A number of conclusions are drawn about variability components with time scales from hours to decades. There are cases when very similar behavior on time scales from several weeks to several months was observed in the presence of substantially different base brightness levels. Both the amplitudes (\( \sim 1\mathop .\limits^m 5\)) and the characteristic shapes and overall durations of these events were the same. The constancy of the amplitudes of these events in magnitudes (i.e., the proportionality of the luminosity in rapid outbursts to the underlying level of the slowly varying component) can be explained in models that associate the variability with changes in the Doppler boosting factor due to variations in the inclination of the relativistic jet to the line of sight, if the slow and rapid components of the optical variability are associated with a steady component of the relativistic jet and shock waves (superluminal components) moving in the jet, respectively. Evidence for such a connection has been found for several superluminal sources. At the same time, there is evidence that in typical IDV events for ON 231, with durations of ∼1.5–2 hr, the amplitudes depend on the base brightness level of the objects, suggesting that the IDV events and less rapid components of the variability have different natures and/or occur in different locations in the source.

Observations of BL Lacertae Object 1803+784 from the Geodetic VLBI Archive of the Washington Correlator

We present 11 maps of BL Lacertae object 1803+784 derived from geodetic VLBI observations archived at the Washington Correlator. The observations were obtained during the years 1988-1999. In this period three superluminal components with an expansion rate of 0.084 mas yr-1 were identified. A remarkable feature of the VLBI images is that the superluminal components only become visible on the jet at a radius of 1 mas from the core, which was interpreted as a result of a helically curved jet. This feature and further evidence of helical patterns found in published high dynamic range VLBI maps at parsec scale and kiloparsec scale enabled us to obtain a well-constrained helical jet. The overall radio morphology of 1803+784 could be described by two dominant components given by a narrow helical jet extending east-west about 6 kpc from the core (deprojected size of 30 kpc) and a much larger north-south component perpendicular to the helical jet (cone axis) with a linear size of about 270 kpc.

The synchrotron spectrum of the M87 jet

We present 1998 HST observations which yield the first single-epoch spectrum of the M87 jet from 0.3 to 2.0 microns. The flattest optical spectra ( α o∼0.6, where F ν ∝ ν −α ; comparable to α ro) are found in two inner jet knots which contain the fastest superluminal components. Other knots have somewhat steeper optical ( α o=0.8–1.1) than radio-optical ( α ro≈0.65) spectra. We do not find significant evidence of near-IR to UV steepening for any component. We combine the HST data with published X-ray and radio data to explore synchrotron emission models and the nature of the jet’s X-ray emission. We find break frequencies ∼10 15–16 Hz for most jet regions, considerably higher than predicted by previous workers. The highest break frequencies are in ‘strong shock’ regions, consistent with possible particle injection or acceleration. Because ‘continuous injection’ synchrotron models overpredict the X-ray emissions by large factors, it is likely that X-ray emitting regions take up only a small fraction of the jet volume.

Changes in the trajectory of the radio jet in 0735+178?

ABSTRA C T We present multi-epoch 8.4- and 43-GHz Very Long Baseline Array images of the BL Lac object 07351178. The images confirm the presence of a twisted jet with two sharp apparent bends of 908 within two milliarcseconds of the core, resembling a helix in projection. The observed twisted geometry could be the result of precession of the jet inlet, but is more likely produced by pressure gradients in the external medium through which the jet propagates. Quasi-stationary components are observed at the locations of the 908 bends, possibly produced by differential Doppler boosting. Identification of components across epochs, since the earliest VLBI observations of this source in 1979.2, proves difficult due to the sometimes large time gaps between observations. One possible identification suggests the existence of superluminal components following non-ballistic trajectories with velocities up to 11:6 ^ 0:6 h 21 65 c. However, in images obtained after mid-1995, components show a remarkable tendency to cluster near several jet positions, suggesting a different scenario in which components have remained nearly stationary in time, at least since mid-1995. Comparison with the earlier published data, covering more than 19 years of observations, suggests a striking qualitative change in the jet trajectory sometime between mid-1992 and mid-1995, with the twisted jet structure with stationary components becoming apparent only at the later epochs. This would require a re-evaluation of the physical parameters estimated for 07351178, such as the observing viewing angle, the plasma bulk Lorentz factor, and those deduced from these.

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.

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.

The Optical–Near‐Infrared Spectrum of the M87 Jet fromHubble Space TelescopeObservations

We present 1998 Hubble Space Telescope observations of M87 that yield the first single-epoch optical and radio-optical spectral index image of the jet at 015 resolution. We find αro ≈ 0.67, comparable to previous measurements, and αo ≈ 0.9 (Fν ∝ ν-α), slightly flatter than previous workers. Reasons for this discrepancy are discussed. These observations reveal a large variety of spectral slopes. Bright knots exhibit significantly flatter spectra than interknot regions. The flattest spectra (αo ~ 0.5-0.6, comparable to or flatter than αro) are found in the two inner jet knots (D-East and HST-1), which contain the fastest superluminal components. The flux maximum regions of other knots have αo ~ 0.7-0.9. The maps of αo and αro appear poorly correlated. In knots A, B, and C, αo and αro are essentially anticorrelated with one another. Near the flux maxima of two inner jet knots (HST-1 and F), changes in αro appear to lag changes in αo, but in two other knots (D and E), the opposite relationship is observed. This is further evidence that the radio and optical emissions of the M87 jet come from substantially different physical regions. The delays observed in the inner jet are consistent with localized particle acceleration in the knots, with tacc tcool for optically emitting electrons in knots HST-1 and F, and tacc ~ tcool for optically emitting electrons in knots D and E. Synchrotron models fit to the radio-optical data yield νB 1016 Hz for knots D, A, and B, and somewhat lower values, νB ~ 1015-1016 Hz, in other regions of the jet. If the X-ray emissions from knots A, B, and D are cospatial with the optical and radio emission, we can strongly rule out the continuous injection model, which overpredicts the X-ray emissions by large factors. Because of the short lifetimes of X-ray synchrotron-emitting particles, the X-ray emission likely traces sites of particle acceleration and fills volumes much smaller than the optical emission regions.

Kinematics of the Parsec‐Scale Relativistic Jet in Quasar 3C 279: 1991–1997

We present results of long-term high-frequency VLBI monitoring of the relativistic jet in 3C 279, consisting of 18 epochs at 22 GHz from 1991 to 1997 and 10 epochs at 43 GHz from 1995 to 1997. Three major results of this study are apparent speeds measured for six superluminal components range from 4.8c to 7.5c (H0 = 70 km s-1 Mpc-1, q0 = 0.1), variations in the total radio flux are due primarily to changes in the VLBI core flux, and the uniform-sphere brightness temperature of the VLBI core is ~1 × 1013 K at 22 GHz after 1995, one of the highest direct estimates of a brightness temperature. If the variability brightness temperature measured for 3C 279 by Lahteenmaki & Valtaoja is an actual value and not a lower limit, then the rest-frame brightness temperature of 3C 279 is quite high and limited by inverse Compton effects rather than equipartition. The parsec-scale morphology of 3C 279 consists of a bright, compact VLBI core, a jet component (C4) that moved from ~2 to ~3.5 mas from the core during the course of our monitoring, and an inner jet that extends from the core to a stationary component, C5, at ~1 mas from the core. Component C4 followed a curved path, and we reconstruct its three-dimensional trajectory using polynomial fits to its position versus time. Component C5 faded with time, possibly due to a previous interaction with C4 similar to interactions seen in simulations by Gomez et al. Components in the inner jet are relatively short lived and fade by the time they reach ~1 mas from the core. The components have different speeds and position angles from each other, but these differences do not match the differences predicted by the precession model of Abraham & Carrara. Although VLBI components were born about six months prior to each of the two observed γ-ray high states, the sparseness of the γ-ray data prevents a statistical analysis of possible correlations.

Parsec‐Scale Blazar Monitoring: Proper Motions

We present proper motions obtained from a dual-frequency, six-epoch, VLBA polarization experiment monitoring a sample of 12 blazars. The observations were made at 15 and 22 GHz at bimonthly intervals over 1996. A total of 10 of the 11 sources for which proper motion could be reliably determined are superluminal. Only J2005+77 has no superluminal components. Three sources (OJ 287, J1224+21, and J1512-09) show motion faster than 10h-1c, requiring γpattern of at least 10h-1 (H0 = 100h km s-1 Mpc-1). We compare our results to those in the literature and find motions outside the previously observed range for four sources. While some jet components exhibit significant nonradial motion, most motion is radial. In at least two sources there are components moving radially at significantly different structural position angles. In five of six sources (3C 120, J1224+21, 3C 273, 3C 279, J1512-09, and J1927+73) that have multiple components with measurable proper motion, the innermost component is significantly slower than the others, suggesting that acceleration occurs in the jet. In the motions of individual components we observe at least one decelerating motion and two "bending" accelerations that tend to align their motions with larger scale structure. We also discuss in detail our procedures for obtaining robust kinematical results from multifrequency VLBI data spanning several epochs.

Jet Stability and the Generation of Superluminal and Stationary Components

We present a numerical simulation of the response of an expanding relativistic jet to the ejection of a superluminal component. The simulation has been performed with a relativistic time-dependent hydrodynamical code from which simulated radio maps are computed by integrating the transfer equations for synchrotron radiation. The interaction of the superluminal component with the underlying jet results in the formation of multiple conical shocks behind the main perturbation. These trailing components can be easily distinguished because they appear to be released from the primary superluminal component instead of being ejected from the core. Their oblique nature should also result in distinct polarization properties. Those appearing closer to the core show small apparent motions and a very slow secular decrease in brightness and could be identified as stationary components. Those appearing farther downstream are weaker and can reach superluminal apparent motions. The existence of these trailing components indicates that not all observed components necessarily represent major perturbations at the jet inlet; rather, multiple emission components can be generated by a single disturbance in the jet. While the superluminal component associated with the primary perturbation exhibits a rather stable pattern speed, trailing components have velocities that increase with distance from the core but move at less than the jet speed. The trailing components exhibit motion and structure consistent with the triggering of pinch modes by the superluminal component. The increase in velocity of the trailing components is an indirect consequence of the acceleration of the expanding fluid, which is assumed to be relativistically hot; if observed, such accelerations would therefore favor an electron-positron (as opposed to proton rest mass) dominated jet.

Internal shock model for the radio emission of Microquasars

The superluminal radio components observed in microquasars are usually interpreted as individual ejection events. We discuss some of the shortcomings of this model and propose the internal shock model in quasi-continuous relativistic jets as an alternative. This model can resolve the problems with the single ejection model and is in agreement with radio observations. We outline some further testable model predictions some of which are already confirmed by observations.

Hydrodynamical and Emission Simulations of Relativistic Jets: Stability and Generation of Superluminal and Stationary Components

We present 2D hydrodynamical and emission simulations of the jet stabilityafter the introduction of strong perturbations on a relativistic jet. These simulations show that the interaction of a single strong perturbation with the underlying jet results in the formation of multiple conical shocks with very specific observational properties.

The Quasar 3C 345 at the Highest Resolution with mm-and Space-VLBI

The quasar 3C345 is one of the best examples of extra-galactic jet with superluminal components moving on helical paths. To get better constraints on the jet physics, we have imaged the source at the highest possible resolution, using mm-VLBI and space-VLBI observations. We observed 3C 345 at 5 different epochs so far (1 epoch with 3-mm ground VLBI, and 4 epochs with space VLBI at 6cm and 18 cm). These measurements expand significantly our ongoing monitoring of 3C 345. In this contribution, we discuss the properties of the jet and its moving components in the immediate vicinity of the core (< 3 mas).

The Compact Core-Jet Region of the Superluminal Quasar 3C 216

Abstract Space very long baseline interferometry (VLBI) observations of the quasar 3C 216 with the VLBI Space Observatory Programme (VSOP) reveal that the parsec scale structure of the source can be well described by compact jet models developed for interpreting the core region of radio-loud active galactic nuclei (AGN). The measured brightness temperature of TB = 7.9 × 1011 K is comparable to the inverse Compton limit, from which we determine a lower limit of δ ∼ 3 for the Doppler-boosting factor. The apparent transverse velocity of the superluminal component is βapp = (3.0±0.2) h−1, assuming a constant velocity, but deceleration of the jet material cannot be excluded from our data. A combination of the above values indicates that the viewing angle of the core-jet to the line-of-sight is less than 19°, and the jet Lorentz-factor exceeds 2.4. The observed small size of the source is probably caused by both an interaction with the interstellar medium and a projection effect.