Superluminal Motion Research Articles

The milliarcsecond-scale jet of PKS 0735+178 during quiescence

We present polarimetric 5 GHz to 43 GHz VLBI observations of the BL Lacertae object PKS 0735+178, spanning March 1996 to May 2000. Comparison with previous and later observations suggests that the overall kinematic and structural properties of the jet are greatly influenced by its activity. Time intervals of enhanced activity, as reported before 1993 and after 2000 by other studies, are followed by highly superluminal motion along a rectilinear jet. In contrast the less active state in which we performed our observations, shows subluminal or slow superluminal jet features propagating through a twisted jet with two sharp bends of about 90° within the innermost three-milliarcsecond jet structure. Proper motion estimates from the data presented here allow us to constrain the jet viewing angle to values ≤ , and the bulk Lorentz factor to be between 2 and 4.

Milliarcsecond-Scale Structure in the Gamma-Ray Loud Quasar PKS 1622−297

Abstract We made a high-resolution VLBI observation of the gamma-ray loud quasar PKS 1622$-$297 with the HALCA spacecraft and ground radio telescopes at 5 GHz in 1998 February, almost 3 yr after the source exhibited a spectacular GeV gamma-ray flare. The source shows an elongated structure toward the west on the parsec scale. The visibility data are well modeled by three distinct components: a bright core and two weaker jet components. Comparison with previous observations confirms that the jet components have an apparent superluminal motion up to $12.1 \,h^{-1} \, c$, with the inner jet components having lower superluminal speeds. We applied the inverse Compton catastrophe model and derived a Doppler factor, $\delta$, of 2.45, which is rather lower than those of other gamma-ray loud active galactic nuclei (AGNs), suggesting that the source was in a more quiescent phase at the epoch of our observation. As an alternative probe of the subparsec-scale structure, we also present the results from multiepoch ATCA total flux monitoring, which indicate the presence of persistent intraday variability consistent with refractive interstellar scintillation. We examined the gamma-ray emission mechanism in light of these observations.

Intrinsic Brightness Temperatures of AGN Jets

We present a new method for studying the intrinsic brightness temperatures of the parsec-scale jet cores of Active Galactic Nuclei (AGN). Our method uses observed superluminal motions and observed brightness temperatures for a large sample of AGN to constrain the characteristic intrinsic brightness temperature of the sample as a whole. To study changes in intrinsic brightness temperature, we assume that the Doppler factors of individual jets are constant in time as justified by their relatively small changes in observed flux density. We find that in their median-low brightness temperature state, the sources in our sample have a narrow range of intrinsic brightness temperatures centered on a characteristic temperature, T_int = 3 x 10^10 K, which is close to the value expected for equipartition, when the energy in the radiating particles equals the energy stored in the magnetic fields. However, in their maximum brightness state, we find that sources in our sample have a characteristic intrinsic brightness temperature greater than 2 x 10^11 K, which is well in excess of the equipartition temperature. In this state, we estimate the energy in radiating particles exceeds the energy in the magnetic field by a factor of ~ 10^5. We suggest that the excess of particle energy when sources are in their maximum brightness state is due to injection or acceleration of particles at the base of the jet. Our results suggest that the common method of estimating jet Doppler factors by using a single measurement of observed brightness temperature and/or the assumption of equipartition may lead to large scatter or systematic errors in the derived values.

A unified accretion-ejection paradigm for black hole X-ray binaries

We present a new picture for the central regions of Black Hole X-ray Binaries. In our view, these central regions have a multi-flow configuration which consists in (1) an outer standard accretion disc down to a transition radius rJ; (2) an inner magnetized accretion disc below rJ driving (3) a non relativistic self-collimated electron-proton jet surrounding, when adequate conditions for pair creation are met; (4) a ultra relativistic electron-positron beam. This accretion-ejection paradigm provides a simple explanation to the canonical spectral states, from radio to X/γ-rays, by varying the transition radius rJ and disc accretion rate independently. Large values of rJ correspond to the Quiescent state for low and the Hard state for larger . These states are characterized by the presence of a steady electron-proton MHD jet emitted by the disc below rJ. The hard X-ray component is expected to form at the jet basis. When rI becomes smaller than the marginally stable orbit ri, the whole disc resembles a standard accretion disc with no jet, characteristic of the Soft state. Intermediate states correspond to situations where . At large , an unsteady pair cascade process is triggered within the jet axis, giving birth to flares and ejection of relativistic pair blobs. This would correspond to the luminous intermediate state, sometimes referred to as the Very High state, with its associated superluminal motions. The variation of rJ independently of is a necessary ingredient in this picture. It arises from the presence of a large scale vertical magnetic field threading the disc. Features such as possible hysteresis and the presence of quasi-periodic oscillations would naturally fit within this new framework.

Large-scale motion, oscillations and a possible halo on the counter-jet side in 1803+784

We present world-array VLBI observations of the blazar 1803+784 performed on May 29, 1993 at A = 18 cm. A 17-station VLBI array, the phased VLA and the 7-station MERLIN array observed the source simultaneously for 11 h. We present the global VLBI map, the MERLIN map, and combined-array maps at different resolutions giving an overview of the morphology of 1803+784 from the inner 10 mas to ∼2 arcsec. We show that the jet bends by around 90° at a core separation of about 0.5 arcsec towards the South. On larger scales, a Westerbork Synthesis Radio Telescope (WSRT) map made at a wavelength of 6 cm resolves the bridge connecting the core with the secondary component some 45 to the south. Wiggles in the ridge of this emission suggest that, as on the 100-pc scale, the jet may also oscillate on 50-100 kpc scales. In addition to improved imaging of the southern component, the 6 cm map provides evidence for amorphous emission to the north, as well as an extended, halo-like component about the nucleus. We compare the source structure at 6 and 18 cm with the structure at 2 cm in a VLBA image obtained on November 6. 1999. Jet wiggling is clearly seen on all scales between 1 mas and 50 arcsec. We find some indication for apparent superluminal motion between ∼5c and 7c for the 25 mas jet component in model-fit results for five epochs of VLBI observations performed at 6 and 18 cm. In addition, we find some evidence for further so-called stationary components in the jet of this source which will have to be confirmed in future observations spaced more densely in time. We discuss the self-similar source morphology observed from mm to cm wavelengths - namely the wiggling on all observed scales - and the implications of the deviations from self-similarity. This is the second of two papers describing the radio structure of S5 1803+784. The first presented the results of 6 years of VLBI monitoring in the X-band (Britzen et al. 2005, MNRAS, in press).

PKS 1622-253: A Weakly Accreting, Powerful Gamma-Ray Source

In this Letter, we discuss new deep radio observations of PKS 1622-253 and their implications for the energetics of the central engine that powers this strong high energy gamma-ray source. Combining archival infrared and optical measurements with new millimeter observations, we show that even though the accretion flow in PKS 1622-253 is under-luminous by quasar standards, a powerful super-luminal jet is launched with a higher kinetic luminosity than most EGRET blazars. Only a few percent of the total jet kinetic luminosity is required to power even the most powerful gamma ray flares that are observed. The implication is that a high accretion system is not required to power the strongest high energy gamma ray sources.

The radio structure of S5 1803+784

The BL Lacertae object S5 1803+784 has been monitored with very long baseline interferometry at λ = 3.6 cm in 43 epochs between 1986.21 and 1993.95. The motivation of this work is to obtain statistically meaningful data with which to study the short-term structural variability in the source on monthly time-scales. We present a detailed analysis of the structural evolution seen in the milliarcsecond-scale jet by way of model-fitting results. Within the simplest identification scenario, the jet can be described as a bent chain of seven jet components within 0.2- and 3-mas separation from the core. New components seem to emerge almost regularly (every 2 yr) from the core. Three jet components approach the brightest and so-called 'stationary' component (at ∼1.4 mas at 8.4 GHz) with apparent superluminal motion of 8-11c. In this paper, we show that the 'stationary' component (under the assumption that this is the brightest jet component in each epoch) oscillates, and we discuss several possible explanations. We show that a reconfinement shock can easily reproduce the observed most recent oscillation of the 'stationary' component. We present evidence that the average jet ridge line is significantly curved and discuss the possibility that the jet components follow a helically bent path.

Coordinated Multiwavelength Observation of 3C 66A during the WEBT Campaign of 2003–2004

The BL Lac object 3C 66A was the target of an extensive multiwavelength monitoring campaign from 2003 July through 2004 April (with a core campaign from 2003 September to 2003 December) involving observations throughout the electromagnetic spectrum. Radio, infrared, and optical observations were carried out by the WEBT-ENIGMA collaboration. At higher energies, 3C 66A was observed in X-rays (RXTE), and at very high energy (VHE) in γ-rays (STACEE, VERITAS). In addition, the source has been observed with the VLBA at nine epochs throughout the period 2003 September to 2004 December, including three epochs contemporaneous with the core campaign. A gradual brightening of the source over the course of the campaign was observed at all optical frequencies, culminating in a very bright maximum around 2004 February 18. The WEBT campaign revealed microvariability with flux changes of ~5% on timescales as short as ~2 hr. The source was in a relatively bright state, with several bright flares on timescales of several days. The spectral energy distribution (SED) indicates a νFν peak in the optical regime. A weak trend of optical spectral hysteresis with a trend of spectral softening throughout both the rising and decaying phases has been found. On longer timescales, there appears to be a weak indication of a positive hardness-intensity correlation for low optical fluxes, which does not persist at higher flux levels. The 3-10 keV X-ray flux of 3C 66A during the core campaign was historically high and its spectrum very soft, indicating that the low-frequency component of the broadband SED extends beyond ~10 keV. No significant X-ray flux and/or spectral variability was detected. STACEE and Whipple observations provided upper flux limits at >150 and >390 GeV, respectively. The 22 and 43 GHz data from the three VLBA epochs made between 2003 September and 2004 January indicate a rather smooth jet with only very moderate internal structure. Evidence for superluminal motion (8.5 ± 5.6 h-1 c) was found in only one of six components, while the apparent velocities of all other components are consistent with 0. The radial radio brightness profile suggests a magnetic field decay ∝r-1 and, thus, a predominantly perpendicular magnetic field orientation.

Relativistic jet motion in the core of the radio-loud quasar J1101+7225

Multi-epoch GHz Very Long Baseline Interferometry (VLBI) data of the radio-loud quasar J1101+7225 were analyzed to estimate the proper motion of extended optically thin jet components. Two components separated from the core could be mapped at 1.66 GHz, which is consistent with earlier observations. In one case we found evidence of high apparent superluminal motion (beta_app= 22.5+/-4) at large (deprojected) distances to the core (22 mas ~ 4 kpc at z= 1.46). Typically in other quasars such high separation velocities are only found much closer to the core component. Furthermore the Doppler factor, the magnetic field strength, and the angular size of the optically thick core were derived using published X-ray data. Analysis of 5 GHz VLBI data reveals the existence of further jet components within the central 5 mas. Additionally the data published so far on the GHz-spectrum were discussed at all angular resolutions. J1101+7225 turns out to be a standard quasar for studying different aspects of radio jet kinematics out to kpc-scales.

Superluminal apparent motions in distant radio sources

We derive the prediction of the standard model of superluminal radio sources for the apparent transverse velocity of a radio source located at redshift z. The apparent velocity of the source is reduced by a factor of 1+z compared to that of a similar nearby source. The cause of this reduction is recession of the distant source due to the expansion of the universe. The apparent velocity of a source can be estimated from its redshift and proper motion using the values of the Hubble constant and the mean densities of different energy components in the universe. We derive an expression for the velocity valid for the currently favored cosmological model: a flat universe with a nonzero cosmological constant.

Anticorrelated Hard X‐Ray Time Lag in GRS 1915+105: Evidence for a Truncated Accretion Disk

Multiwavelength observations of Galactic black hole candidate sources indicate a close connection between the accretion disk emission and the jet emission. The recent discovery of an anticorrelated time lag between the soft and hard X-rays in Cygnus X-3 (Choudhury & Rao) constrains the geometric picture of the disk-jet connection into a truncated accretion disk, the truncation radius being quite close to the black hole. Here we report the detection of similar anticorrelated time lag in the superluminal jet source GRS 1915+105. We show the existence of pivoting in the X-ray spectrum during the delayed anticorrelation, and we also find that the quasi-periodic oscillation frequency changes along with the spectral pivoting. We explore theoretical models to understand this phenomenon.

The extreme flare in III Zw 2:

A very detailed monitoring of a radio flare in the Seyfert I galaxy III Zw 2 with the VLA and the VLBA is presented. The relative astrometry in the VLBA observations was precise to a few muas. The spectral and spatial evolutions of the source are closely linked, and these observations allowed us to study a textbook example of a synchrotron self-absorbed jet in great detail. We observed a phase where the jet gets frustrated with neither expansion nor spectral evolution. Then the jet breaks free and starts to expand with apparent superluminal motion. This expansion is accompanied by a strong spectral evolution. The results are a good confirmation of synchrotron theory and equipartition for jets.

The Center of Activity in the Compact Steep‐Spectrum Superluminal Source 3C 138

We present the results from the first quasi-simultaneous multifrequency (2.3, 5.0, 8.4, and 15 GHz) very long baseline interferometry (VLBI) observations of the compact steep-spectrum (CSS) superluminal source 3C 138. For the first time, the spectral distribution of the components within its central 10 milliarcsecond (mas) region was obtained. This enables us to identify the component at the western end as the location of the nuclear activity, assuming that the central engine is associated with one of the detected components. The possibility that none of these visible components is the true core is also discussed. The new measurements further clarify the superluminal motions of its inner jet components. The multifrequency data reveal a convex spectrum in one jet component, implying the existence of free-free absorption by the ambient dense plasma.

Kinematic study of the blazar S5 0716+714

We present the results of a multi-frequency study of the structural evolution of the VLBI jet in the BL Lac object 0716+714 over the last 10 years. We show VLBI images obtained at 5 GHz, 8.4 GHz, 15 GHz and 22 GHz. The milliarcsecond source structure is best described by a one-sided core-dominated jet of ∼10 mas length. Embedded jet components move su- perluminally with speeds ranging from 5 c to 16 c (assuming z = 0.3). Such fast superluminal motion is not typical of BL Lac objects, however it is still in the range of jet speeds typically observed in quasars (10 c to 20 c). In 0716+714, younger com- ponents that were ejected more recently seem to move systematically slower than the older components. This and a systematic position angle variation of the inner (1 mas) portion of the VLBI jet suggests an at least partly geometric origin of the observed velocity variations. The observed rapid motion and the derived Lorentz factors are discussed with regard to the rapid Intra-Day Variability (IDV) and the γ-ray observations, from which very high Doppler factors are inferred.

Relativistic phenomena in active galactic nuclei

The idea that the radio jets in AGN contain material in relativistic motion is supported by many lines of observational evidence, including morphology, brightness temperature estimated with interferometers and with intrinsic variations, interstellar scintillations, X-rays, and superluminal motion. These are largely independent, and taken together make an irrefutable case for relativistic motion.

Superluminal Motion and Polarization in Blazars

A relativistic beaming model has been successfully used to explain the observed properties of active galactic nuclei (AGNs). In this model there are two emission components, a boosted one and an unbeamed one, shown up in the radio band as the core and lobe components. The luminosity ratio of the core to the lobe is defined as the core-dominance parameter (R = { L Core }/{ L Lobe }). The de-beamed radio luminosity ( L db jet ) in the jet is assumed to be proportional to the unbeamed luminosity ( L ub ) in the co-moving frame, i.e., f = { L db jet }/{ L ub }, and f is determined in our previous paper. We further discuss the relationship between BL Lacertae objects (BLs) and flat spectrum radio quasars (FSRQs), which are subclasses of blazars with different degrees of polarization, using the calculated values of the ratio f for a sample of superluminal blazars. We found 1) that the BLs show smaller averaged Doppler factors and Lorentz factors, larger viewing angles and higher core-dominance parameters than do the FSRQs, and 2) that in the polarization-core dominance parameter plot (P-log R) the BLs and FSRQs occupy a scattered region, but in a revised plot (log {P}/{c(m)}-log R), they gather around two different lines, suggesting that they have some different intrinsic properties.

The Linear Accelerating Superluminal Motion Model

Accelerating superluminal motions were detected recently by multiepoch very long baseline interferometry observations. Here a linear accelerating superluminal motion (LASM) model is proposed to interpret the observed phenomena. The model provides a direct and accurate way to estimate the viewing angle of a relativistic jet. It also predicts that both Doppler boosting and deboosting effects may take place in an accelerating forward jet. The LASM model is applied to the data of the quasar 3C 273, and the initial velocity, acceleration, and viewing angle of its three components are derived through model fits. The variations of the viewing angle suggest that a supermassive black hole binary system may exist in the center of 3C 273. The gap between the inner and outer jet in some radio-loud active galactic nuclei my be explained in terms of Doppler deboosting effects when the components accelerate to ultrarelativistic speed.

Is GRS 1915+105 A Microquasar?

The large mechanical luminosity of the jets of GRS 1915+105 should give rise to luminous emission regions, similar to those observed in radio galaxies, where the jets interact with the gas surrounding the source. However, no radio synchrotron emission of the expected morphology has been found. Here we present the results of a study suggesting that radio bremsstrahlung from the compressed and heated ISM in front of the jets should be detectable, while the synchrotron lobes may be too faint. We identify these jet impact sites with two well-known IRAS regions. This identification suggests a distance of GRS 1915+105 of $6.5\pm1.6$ kpc, significantly closer than the usually assumed distance of 11 to 12 kpc. We discuss the implications of this reduced distance estimate. The apparent motion of small-scale jet components is not superluminal, so if superluminal motion is required for an object to be termed a microquasar, GRS 1915+105 actually does not qualify. The mass of the black hole in the system is increased to $21\pm9$ M$_{\odot}$ while the mechanical luminosity of the jets is reduced to 14% of the Eddington luminosity.

Relative variations of the physical parameters of variable extragalactic radio sources

A method for studying the physical conditions in compact components of extragalactic radio sources displaying variability on time scales of hundreds of days is proposed. The method can be used to estimate the relative variations of the magnetic-field strength and number density of relativistic electrons in superluminal jets from the cores of quasars and radio galaxies. Results are presented for the jets of the quasars 3C 120, 3C 273, 3C 279, and 3C 345. The energies of the magnetic field and relativistic particles in these objects are not in equipartition. As a rule, the magnetic-field strength decreases appreciably during the evolution of an expanding jet, while the number of relativistic electrons grows.

VLBI imaging and optical variability of the BL Lac object OQ 530 (B1418+546)

Results of VLBI and optical observations are presented for the BL Lac object OQ 530 (B1418+546), whose long term optical variability is characterised by a decreasing mean luminosity trend. EVN images at 1.6 GHz and 5 GHz of OQ 530 show a very bright core with a much weaker jet extending to about 35–40 mas. The radio flux density from the core in June 2001 was much higher than that measured in February 1999. A similar increase was also found from nearly simultaneous optical photometry, while the total flux density of the jet remained unchanged. The structure of the jet shows several knots which move outward with an apparent superluminal motion with a $\beta_{\rm app}\geq3.5h^{-1}$. The region within about 3 mas from the core contains a component not well resolved in our images and emerging from it in the jet direction. That component was also detected in images taken from 1990 to 1997 and it seems to be a rather stable structural feature. Using nearly simultaneous optical and radio data, we can describe the SED of the synchrotron peak with a simple formula, approximating a power law at low frequencies and a log-parabola at the high ones, whose maximum lies in the range $3.5{-}5\times10^{13}$ Hz, and the extrapolation in the X-ray range gives a flux comparable to that observed with BeppoSAX.