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In a previous work, we have identified the spin of the dominant black hole of a binary from its jet properties. Analysing Very Long Baseline Array (VLBA) observations of the quasar S5 1928+738, taken at 15-GHz during 43 epochs between 1995.96 and 2013.06, we showed that the inclination angle variation of the inner (<2 mas) jet symmetry axis naturally decomposes into a periodic and a monotonic contribution. The former emerges due to the Keplerian orbital evolution, while the latter is interpreted as the signature of the spin-orbit precession of the jet emitting black hole. In this paper, we revisit the analysis of the quasar S5 1928+738 by including new 15-GHz VLBA observations extending over 29 additional epochs, between 2013.34 and 2020.89. The extended data set confirms our previous findings which are further supported by the flux density variation of the jet. By applying an enhanced jet precession model that can handle arbitrary spin orientations κ with respect to the orbital angular momentum of a binary supermassive black hole system, we estimate the binary mass ratio as ν = 0.21 ± 0.04 for κ = 0 (i.e. when the spin direction is perpendicular to the orbital plane) and as ν  = 0.32 ± 0.07 for κ = π/2 (i.e. when the spin lies in the orbital plane). We estimate more precisely the spin precession velocity, halving its uncertainty from |$(-0.05\pm 0.02)$| to |$(-0.04\pm 0.01)^{\circ }\, \mathrm{yr}^{-1}$|⁠. [ABSTRACT FROM AUTHOR]

This study aims to characterize, for the first time, intranight optical variability (INOV) of low-mass active galactic nuclei (LMAGNs) which host a black hole (BH) of mass MBH ∼ 106 M ⊙, i.e. even less massive than the Galactic centre BH Sgr A* and 2–3 orders of magnitude below the supermassive black holes (SMBHs, |$M_{BH}\, \sim \, 10^8 \rm{-} 10^9 M_{\odot }$|⁠), which are believed to power quasars. Thus, LMAGNs are a crucial subclass of AGNs filling the wide gap between SMBH and stellar-mass BHs of Galactic X-ray binaries. We have carried out a 36-session campaign of intranight optical monitoring of a well-defined, representative sample of 12 LMAGNs already detected in X-ray and radio bands. This set of LMAGNs is found to exhibit INOV at a level statistically comparable to that observed for blazars (MBH ≳ 108–9 M ⊙) and for the γ-ray-detected Narrow-line Seyfert1 galaxies (MBH ∼ 107 M ⊙) which, too, are believed to have relativistic jets. This indicates that the blazar-level activity can even be sustained by central engines with BHs near the upper limit for intermediate-mass BHs (⁠|$M_{BH}\, \sim \, 10^3 \rm{-}10^6 M_{\odot }$|⁠). [ABSTRACT FROM AUTHOR]

We report the first study to characterize intranight variability of the blazar class from the perspective of (rest-frame) ultraviolet (UV) emission. For this, we carried out intranight optical monitoring of 14 flat-spectrum radio quasars (FSRQs) located at high redshifts (1.5 < z < 3.7), in 42 sessions of median duration ∼5.4 h. These sources were grouped into two samples distinguished by published fractional optical polarization: (i) nine low-polarization sources with |$p_{{\rm opt}} \lt 3{{\ \rm per\ cent}}$| and (ii) five high-polarization sources. Unexpectedly, a high duty cycle (DC ∼ 30 per cent) is found for intranight variability (with amplitude |$\psi\, \gt\, 3{{\ \rm per\ cent}}$|⁠) of the low-polarization sources. This DC is a few times higher than that reported for low-polarization FSRQs located at moderate redshifts (⁠|$z\, \sim$| 0.7) and hence typically monitored in the rest-frame blue-optical. Furthermore, we found no evidence for an increased intranight variability of UV emission with polarization, in contrast to the strong correlation found for intranight variability of optical emission. We briefly discuss this in the context of an existing scenario which posits that the non-thermal UV emission of blazars arises from a relativistic particle population different from that radiating up to near-infrared/optical frequencies. [ABSTRACT FROM AUTHOR]

VLBI observations of the extremely [gamma]-bright blazar PKS 0528 + 134 at 8, 22, 43, and 86 GHz reveal a strongly bent one-sided-core jet structure with at least three moving and two apparently stationary jet components. At the highest observing frequencies the brightest and most compact jet component (the VLBI core) is unresolved with an upper limit to its size of [double tilde] 50 microarcsec corresponding to [double tilde] 0.2 parsec [[H.sub.0 = 100 km.[s.sup.-1].[Mpc.sup.-1] ([megaparsec.sup.-1]), [q.sub.0] = 0.5, where [H.sub.0] is Hubble constant and [q.sub.0] is the deceleration parameter]. Two 86-GHz VLBI observations performed in 1993.3 and 1994.0 reveal anew jet component emerging with superluminal speed from the core. Linear back-extrapolation of its motion yields strong evidence that the ejection of this component is related to an outbutst in the millimeter regime and a prec.eding intense flare of they [gamma]-flux density observed in early 1993. This and the radio / optical 'light curves' and VLBI data for two other sources (S5 0836 + 710 and 3C 454.3) suggest that the observed [gamma]-radiation might be Doppler-boosted and perhips is closely related to the physical processes acting near the 'base' of the highly relativistic jets observed in quasars.

On 2019/07/30.86853 ut , IceCube detected a high-energy astrophysical neutrino candidate. The Flat Spectrum Radio Quasar PKS 1502+106 is located within the 50 per cent uncertainty region of the event. Our analysis of 15 GHz Very Long Baseline Array (VLBA) and astrometric 8 GHz VLBA data, in a time span prior and after the IceCube event, reveals evidence for a radio ring structure that develops with time. Several arc-structures evolve perpendicular to the jet ridge line. We find evidence for precession of a curved jet based on kinematic modelling and a periodicity analysis. An outflowing broad line region (BLR) based on the C  iv line emission (Sloan Digital Sky Survey) is found. We attribute the atypical ring to an interaction of the precessing jet with the outflowing material. We discuss our findings in the context of a spine-sheath scenario where the ring reveals the sheath and its interaction with the surroundings (narrow line region, NLR, clouds). We find that the radio emission is correlated with the γ-ray emission, with radio lagging the γ-rays. Based on the γ-ray variability time-scale, we constrain the γ-ray emission zone to the BLR (30–200  r g) and within the jet launching region. We discuss that the outflowing BLR provides the external radiation field for γ-ray production via external Compton scattering. The neutrino is most likely produced by proton–proton interaction in the blazar zone (beyond the BLR), enabled by episodic encounters of the jet with dense clouds, i.e. some molecular cloud in the NLR. [ABSTRACT FROM AUTHOR]

Context. Studies of periodic and quasi-periodic phenomena in optical and radio bands are important for understanding the physical processes in quasars. Investigation of periodic/quasi-periodic behavior of the relativistic jets in blazars is particularly significant because it can provide unique information about the formation, collimation, and acceleration of the jets and the properties of the central engines (black hole/accretion disk systems) in blazars. Aims. We investigate the parsec-scale kinematics of the 31 superluminal components observed in blazar 3C279 and attempt to search for evidence of its jet precession and double-jet structure. Methods. The previously suggested precessing jet nozzle model is applied to model-fit the kinematics of its superluminal components observed during the 1981–2015 period. It is shown that the parsec-scale kinematics of the entire source can be interpreted in terms of a double-jet scenario. Results. The superluminal components observed in 3C279 can be divided into two groups that are ejected from two relativistic jets. The two jets have different orientations in space and jet-cone shapes, but both jets precess with the same precession period of 25 yr (16.3 yr in the source frame). The kinematic features of all the superluminal knots (trajectory, core separation, and apparent velocity) can be consistently explained. Their innermost trajectories follow the respective precessing common parabolic patterns with trajectory curvatures that occurred in the outer jet regions at different core separations. The bulk Lorentz factor, Doppler factor, and viewing angle of their motion are derived. The unusual jet-direction change of ∼100° observed in 2010–2011 can be naturally explained. Conclusions. We propose a double-jet structure scenario for 3C279 and suggest that there may be a supermassive black hole binary in the center of 3C279 ejecting two precessing relativistic jets, resulting in its very complex structure and kinematics on parsec scales, and with extremely variable emission across the electromagnetic spectrum. Because the two jets have the same precession period, the precession of the double jet may have originated from the modulation of their jet orientation by the change in their orbital velocity direction relative to the observer. In this case the mass ratio m/M of the binary is approximately equal to the ratio of the jet cone widths, being on the order of ∼0.5. [ABSTRACT FROM AUTHOR]

OJ287 is the best candidate active galactic nucleus (AGN) for hosting a supermassive binary black hole (SMBBH) at very close separation. We present 120 Very Long Baseline Array (VLBA) observations (at 15 GHz) covering the time between April 1995 and April 2017. We find that the OJ287 radio jet is precessing on a time-scale of ∼22 yr. In addition, our data are consistent with a jet-axis rotation on a yearly time-scale. We model the precession (24 ± 2 yr) and combined motion of jet precession and jet-axis rotation. The jet motion explains the variability of the total radio flux-density via viewing angle changes and Doppler beaming. Half of the jet-precession time-scale is of the order of the dominant optical periodicity time-scale. We suggest that the optical emission is synchrotron emission and related to the jet radiation. The jet dynamics and flux-density light curves can be understood in terms of geometrical effects. Disturbances of an accretion disc caused by a plunging BH do not seem necessary to explain the observed variability. Although the SMBBH model does not seem necessary to explain the observed variability, an SMBBH or Lense-Thirring precession (disc around single BH) seem to be required to explain the time-scale of the precessing motion. Besides jet rotation also nutation of the jet axis could explain the observed motion of the jet axis. We find a strikingly similar scaling for the time-scales for precession and nutation as indicated for SS433 with a factor of roughly 50 times longer in OJ287. [ABSTRACT FROM AUTHOR]

Context. The study of periodic (or quasi-periodic) variabilities in optical and radio bands and quasi-periodic radio-jet swings are important to further our understanding of the physical processes in blazars. Among these the correlation between the periodic or quasi-periodic phenomena in radio and optical bands is particularly significant, because it can provide unique information about the relativistic jets and central engines in the nuclei of blazars. Aims. We aim to investigate the possibility that the radio jet swing on parsec scales observed in PG 1302-102 (z = 0.278) is a quasi-periodic phenomenon and study its correlation with the periodic optical variability claimed in a recently published work, seeking evidence for a binary black hole system. Methods. The precessing jet-nozzle model proposed in our previous works was applied to simulate the kinematics of the superluminal components. It is shown that the inner-jet kinematic features can well be explained in terms of the precessing nozzle model. Results. Based on the model simulation (model fitting) of the inner kinematics for its six superluminal components, a precession period of ~5.1583 ± 0.5 yr is derived for the radio jet swing and the kinematics of all the six components are consistently interpreted. The similarity between the radio jet precession period and the optical period found in its optical light curve may be physically significant. Both periodic behaviors in radio and optical bands could be explained in terms of the orbital motion of a black hole binary, if the orbital plane makes large inclinations to the sky plane: the orbital motion of the primary hole produces the periodic jet swing and the orbital motion of the secondary hole produces the periodic optical variability as suggested in the literature. Thus the total mass and the mass ratio of the binary are estimated. Conclusions. Based on this analysis, we show that PG 1302-102 might have a supermassive black hole binary existing in its nucleus and it is starting to enter its inspiral phase of merging. Gravitational radiation would start to dominate the energy-momentum loss for its orbital shrinkage. [ABSTRACT FROM AUTHOR]

We present a detailed analysis of 30 very long baseline interferometric (VLBI) observations of the BL Lac object S5 1803+784 (z= 0.679), obtained between mean observational time 1994.67 and 2012.91 at observational frequency 15 GHz. The long-term behaviour of the jet ridge line reveals the jet experiences an oscillatory motion superposed on its helical jet kinematics on a time-scale of about 6 yr. The excess variance of the positional variability indicates the jet components being farther from the VLBI core have larger amplitude in their position variations. The fractional variability amplitude shows slight changes in 3 yrbins of the component's position. The temporal variability in the Doppler boosting of the ridge line results in jet regions behaving as flaring 'radio lanterns'. We offer a qualitative scenario leading to the oscillation of the jet ridge line that utilizes the orbital motion of the jet emitter black hole due to a binary black hole companion. A correlation analysis implies composite origin of the flux variability of the jet components, emerging due to possibly both the evolving jet structure and its intrinsic variability. [ABSTRACT FROM AUTHOR]

Context: The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for a bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large BH mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. Many kinematic studies have been performed to determine the proper motions in the jet. Despite M87 providing all proofs of being an active BH, the apparent jet speed remained puzzling, because proper motion measurements between 15 and 43 GHz for the same region of 1-10 mas core distance provided largely discrepant results. This source is a prime object to be studied in exquisite detail with the upcoming Event Horizon Telescope (EHT) observations because it promises to allow a direct view on the jet launching process itself. Aims: We aim to decipher some of the kinematic puzzles in the pc-scale jet with the analysis we present here. Methods: We re-modeled and re-analyzed 31 VLBA observations at 15 GHz obtained within the MOJAVE program. The data span a time range between Jul. 1995 and May 2011. We performed a detailed investigation of the pc-scale jet kinematics on different timescales, the shortest periods between the observations beeing 10 and 80 days, and in different jet modes, making use of VLBA observations. In addition, we studied the jet ridge line behavior as a function of time. Special care was taken to analyze the region close to the 15 GHz core, and the dynamics and distribution of newly emerging jet features in the jet. Results: We find an indication for apparent superluminal motion in the jet. Moreover, we present evidence for acceleration between 0.5 and 10 mas of core separation. The data suggest that the central part of M87 at 15 GHz seems to be rotating. Jet components and counter-jet components are ejected in different directions under varying angles, explaining the impression of a broad opening angle. In this paper we present evidence for two different operating modes of the jet of M87. The jet switches between two phases: i) the jet ridge line is at least double or the jet axis is displaced vertically, and ii) an unperturbed phase where the jet ridge line remains almost straight but is smoothly curved and the jet components are aligned along a classical jet axis. The mode change occurs every couple of years. Between the two operating modes, a transition phase is visible. Conclusions: The M87 jet visible at 15 GHz probes a different physical zone compared to the standard blazar-zone we tend to see in AGN jets. The most likely scenario explaining the observed phenomena is a turbulent mass loading into the jet, most probably due to local, fast reconnection processes driven by turbulence of a tangled magnetic field, which is either generated in the accretion disk or the disk corona. In addition, on large scales, a global magnetic structure is required to channel the turbulent flow into what evolves into a large-scale jet. Large-scale jet instabilities may explain the curved pattern of the observed jet flow. [ABSTRACT FROM AUTHOR]

The supermassive black hole, Sagittarius A* (Sgr A*), at the centre of the Milky Way undergoes regular flaring activity, which is thought to arise from the innermost region of the accretion flow. Between 2010 and 2014, we performed monitoring observations of the Galactic Centre to study the flux-density variations at 3 mm using the Australia Telescope Compact Array (ATCA). We obtain light curves of Sgr A* by subtracting the contributions from the extended emission around it, and the elevation and time-dependent gains of the telescope. We perform structure function analysis and the Bayesian blocks representation to detect flare events. The observations detect six instances of significant variability in the flux density of Sgr A* in three observations, with variations between 0.5 and 1.0 Jy, which last for 1.5-3 h. We use the adiabatically expanding plasmon model to explain the short time-scale variations in the flux density. We derive the physical quantities of the modelled flare emission, such as the source expansion speed vexp, source sizes, spectral indices and the turnover frequency. These parameters imply that the expanding source components are either confined to the immediate vicinity of Sgr A* by contributing to the corona or the disc, or have a bulk motion greater than vexp. No exceptional flux-density variation on short flare time-scales was observed during the approach and the flyby of the dusty S-cluster object (DSO/G2). This is consistent with its compactness and the absence of a large bow shock. [ABSTRACT FROM AUTHOR]

We investigate the pc-scale kinematics and kpc-scale radio morphology of the quasar PG 1302-102, which may harbour a sub-pc separation supermassive binary black hole system at its centre as inferred from optical variability. High-resolution radio interferometric measurements obtained with the Very Long Baseline Array (VLBA) in the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments programme at 15 GHz at 20 epochs spanning 17 years were analysed to investigate the pc-scale radio structure. Archival observations with the Very Large Array at 1.4 and 5 GHz were obtained to study the kpc-scale morphology. We find that the pc-scale jet is inclined within ~2.° 2 to the line of sight and has a half-opening angle of ~0.° 2. The parameters derived from the pc-scale radio jet are qualitatively consistent with those obtained from the analysis of the optical light curve of PG 1302-102.We obtain at least 0.08 for the mass ratio of the two black holes in the system. We find some indication for a helical jet structure on kpc-scale, but the directions of the inner and the extended radio jets are significantly different, obstructing a straightforward connection of the pc- and kpc-scale jets within the binary scenario. [ABSTRACT FROM AUTHOR]

Detailed and long-term VLBI (Very Long Baseline Interferometry) studies of the variable jets of supermassive black holes helps us to understand the emission processes of these fascinating phenomena. When observed and traced precisely, jet component kinematics reveals details about the potential motion of the jet base. Following this motion over decades with VLBI monitoring reveals - in some cases - the signatures of precession. While several processes can cause precession, the most likely cause seems to be a supermassive binary black hole in the central region of the AGN. We present examples of the analysis of high-resolution VLBI observations which provides us with insight into the physics of these objects and reveals evidence for the presence of double black hole cores. EHT (Event Horizon Telescope) observations will probably soon tell us more about the jet origin and launching mechanism at the very centers of nearby active galactic nuclei. An important question to be addressed by the EHT and related observations will be whether Sgr A*, the supermassive black hole in the Galactic Center, has a jet as well. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Modeling trajectories of VLBI components ejected by the nucleus of 1928+738 shows that the VLBI jet contains three trajectory families, meaning that the VLBI components are ejected from three different origins. The fit of components C1, C6, and C8 indicates that the nucleus of 1928+738 contains two binary black hole systems. The first is associated with the stationary components Cg and CS and is characterized by a radius Rbin,1 ≈ 0:220 mas; both black holes ejected VLBI components almost regularly between 1990 and 2010. The second binary black hole system is not associated with stationary components and is characterized by a radius Rbin,2 ≈ 0:140 mas; it ejected only three VLBI components between 1994 and 1999. The two black hole systems are separated by ≈1.35 mas. We briefly discuss the consequences of the existence of binary black holes systems in radio quasars to make the link between radio quasars and Gaia. [ABSTRACT FROM AUTHOR]

We present a high-frequency very long baseline interferometry (VLBI) kinematical study of the BL Lac object S5 0716+714 over the time period of September 2008 to October 2010. The aim of the study is to investigate the relation of the jet kinematics to the observed broadband flux variability. We find significant non-radial motions in the jet outflow of the source. In the radial direction, the highest measured apparent speed is ~37 c, which is exceptionally high, especially for a BL Lac object. Patterns in the jet flow reveal a roughly stationary feature ~0.15 mas downstream of the core. The long-term fits to the component trajectories reveal acceleration in the sub-mas region of the jet. The measured brightness temperature, TB, follows a continuous trend of decline with distance, TB ∝ rjet-(2.36 ± 0.41), which suggests that there is a gradient in Doppler factor along the jet axis. Our analysis favors that a moving disturbance (or a shock wave) from the base of the jet produces the high-energy (optical to γ-ray) variations upstream of the 7 mm core and then later causes an outburst in the core. Repetitive optical/γ-ray flares and the curved trajectories of the associated components suggest that the shock front propagates along a bent trajectory or helical path. Sharper γ-ray flares could be related to the passage of moving disturbances through the stationary feature. Our analysis indicates that the γ-ray and radio emission regions have different Doppler factors. [ABSTRACT FROM AUTHOR]

Very long baseline interferometry (VLBI) allows for high-resolution and high-sensitivity observations of relativistic jets, that can reveal periodicities of several years in their structure. We perform an analysis of long-term VLBI data of the quasar S5 1928+738 in terms of a geometric model of a helical structure projected onto the plane of the sky. We monitor the direction of the jet axis through its inclination and position angles. We decompose the variation of the inclination of the inner 2 mas of the jet of S5 1928+738 into a periodic term with amplitude of ~0°.89 and a linear decreasing trend with rate of ~0 °.05yr-1. We also decompose the variation of the position angle into a periodic term with amplitude of ~3 °.39 and a linear increasing trend with rate of ~0°.24 yr-1. We interpret the periodic components as arising from the orbital motion of a binary black hole inspiralling at the jet base and derive corrected values of the mass ratio and separation from the accumulated 18 yr of VLBI data. Then, we identify the linear trends in the variations as due to the slow reorientation of the spin of the jet emitter black hole induced by the spin-orbit precession and we determine the precession period TSO = 4852 ± 646 yr of the more massive black hole, acting as the jet emitter. Our study provides indications, for the first time from VLBI jet kinematics, for the spinning nature of the jet-emitting black hole. [ABSTRACT FROM AUTHOR]

Context. Double-lobe radio galaxies are ideally suited to investigate the interaction of the individual components of the radio structure with the intergalactic medium and the interstellar medium of the host galaxy. SDSS J080800.99+483807.7 has been serendipitously discovered inMERLIN 18 cm observations to be a double-lobed radio galaxy. Because it is an optically faint source, basic information like redshift, linear size, and structure has been incomplete until now. Furthermore, there are no spectra of this source available in any databases. Aims. The goal of this work is to derive the main physical properties of SDSS J080800.99+483807.7 and study the possible interaction between the radio jets and the interstellar medium of the host galaxy. Methods. To achieve this goal, we used optical spectroscopy and radio interferometry. The radio data were obtained with MERLIN at 18 cm and the optical data with the Multi-Object Double Spectrograph (MODS) at the Large Binocular Telescope (LBT). Results. The redshift of the galaxy is z = 0.2805 ± 0.0003, resulting in a linear size of the observed radio structure of ~26.3 kpc. The optical line emission as well as the infrared and radio continuum emission suggest a high star-formation activity. In addition, we estimated the mass of the central black hole to be log (MBH/M☉) ≈ 6.9. Conclusions. The MODS spectrum and the optical images from Sloan Digital Sky Survey suggest that SDSS J080800.99+483807.7 is an elliptical host galaxy. In combination with the overall radio structure, we argue that the star formation could be the result of the back-flow along the jet and the interstellar medium of the host. [ABSTRACT FROM AUTHOR]

A fast moving infrared excess source (G2) which is widely interpreted as a core-less gas and dust cloud approaches Sagittarius A* (Sgr A*) on a presumably elliptical orbit. VLT Ks-band and Keck K′-band data result in clear continuum identifications and proper motions of this ∼19m Dusty S-cluster Object (DSO). In 2002-2007 it is confused with the star S63, but free of confusion again since 2007. Its near-infrared (NIR) colors and a comparison to other sources in the field speak in favor of the DSO being an IR excess star with photospheric continuum emission at 2 microns than a core-less gas and dust cloud. We also find very compact L′-band emission (<0.1″) contrasted by the reported extended (0.03″ up to ∼0.2″ for the tail) Brγ emission. The presence of a star will change the expected accretion phenomena, since a stellar Roche lobe may retain a fraction of the material during and after the peri-bothron passage. [ABSTRACT FROM PUBLISHER]

Context. 2251+158 (3C 454.3) is a well-studied quasar with rather unusual properties. It is among the most variable and brightest gamma-ray emitting active galactic nucleus (AGN) in the sky observed by Fermi. The multi-wavelength flux density emission of this source is peculiar, and so is the very long baseline interferometry (VLBI) structure. While it is usually assumed that the jet and jet-components in a given AGN show more or less the same properties (e.g., apparent velocities and paths) with time, we demonstrate here that unusual morphologies (arc-like structure) can occur as temporal phenomena. We also show that the kinematic properties of jet components change from slow to fast apparent motion. Aims. We present the detection of an arc around the core region. To understand the physical nature of this and other peculiar kinematic properties of the parsec-scale jet of 2251+158 we performed detailed radio-interferometric studies of this prototypical AGN. Methods. We (re-)analyzed 32 Very Long Baseline Array (VLBA) observations covering 16 years in time (between 1995.57 and 2011.48), observed at 15 GHz from the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE/2 cm) survey and derived the parameters of the observed VLBI structure. Three 43 GHz VLBA observations from 2001.22, 2005.38, and 2005.76 have been reprocessed. We fitted the components of the VLBI structure with Gaussian components. We studied the properties of light-curves obtained in the radio (4.8, 8.0, and 14.5 GHz) and optical regime (R-band), and studied the correlations between the flaring properties and the VLBI structure of the source. Results. We report the detection of an arc-like structure around the core of an AGN. This ring-structure becomes visible in VLBI maps around 1996. It expands with an apparent proper motion between 0.12 ± 0.01 mas yr-1 and 0.19 ± 0.01 mas yr-1 (corresponding to 5.4 ± 0.3c to 9.0 ± 0.4c) and dominates the parsec-scale structure for about 14 years. In addition, we report a significant change in the kinematic properties of jet components close to the core. A phase of slow apparent radial motion (directed away from the core) of two inner jet components (A and B) and no component ejection after a major radio-flare in 1995 is atypical for AGN. This phase is followed by a more typical behaviour in which the same components separate with higher apparent speeds from the core and two new components appear to be ejected from the central region. Conclusions. We observe significant changes in the morphological and kinematic properties of the parsec-scale jet. Taken together, it is tempting to see a causal connection between the different phenomena. We briefly suggest and discuss several possible physical scenarios to explain the observed phenomena. The kinematic changes as well as the changes in the flaring characteristics could be explained geometrically because of a change in the angle to the line of sight towards the observer assuming a helical structure of the jet. Another possible explanation for the observed phenomena and the correlations among them could be a precessing jet (precession period on timescale of 14.5 years) in combination with its interaction with an external inhomogeneous medium surrounding the core. These and other possible explanations will be investigated further and manuscripts presenting the results are in preparation. [ABSTRACT FROM AUTHOR]

If we assume that nuclei of extragalactic radio sources contain binary black hole systems, the two black holes can eject VLBI components, in which case two families of different VLBI trajectories will be observed. Another important consequence of a binary black hole system is that the VLBI core is associated with one black hole, and if a VLBI component is ejected by the second black hole, one expects to be able to detect the offset of the origin of the VLBI component ejected by the black hole that is not associated with the VLBI core. The ejection of VLBI components is perturbed by the precession of the accretion disk and the motion of the black holes around the center of gravity of the binary black hole system. We modeled the ejection of the component taking into account the two pertubations and present a method to fit the coordinates of a VLBI component and to deduce the characteristics of the binary black hole system. Specifically, this is the ratio Tp=Tb where Tp is the precession period of the accretion disk and Tb is the orbital period of the binary black hole system, the mass ratio M1=M2, and the radius of the binary black hole system Rbin. From the variations of the coordinates as a function of time of the ejected VLBI component, we estimated the inclination angle io and the bulk Lorentz factor γ of the modeled component. We applied the method to component S1 of 1823+568 and to component C5 of 3C 279, which presents a large offset of the space origin from the VLBI core. We found that 1823+568 contains a binary black hole system whose size is Rbin ≈ 60 μas (μas is a microarcsecond) and 3C 279 contains a binary black hole system whose size is Rbin ≈ 420 μas. We calculated the separation of the two black holes and the coordinates of the second black hole from the VLBI core. This information will be important to link the radio reference-frame system obtained from VLBI observations and the optical reference-frame system obtained from Gaia. [ABSTRACT FROM AUTHOR]

A new paradigm for active galactic jet kinematics has emerged through detailed investigations of BL Lac objects using very long baseline radio interferometry. In this new scheme, most, if not all, jet components appear to remain stationary with respect to the core but show significant non-radial motions. This paper presents results from our kinematic investigation of the jets of a statistically complete sample of radio-loud flat-spectrum active galaxies, focusing on the comparison between the jet kinematic properties of BL Lacs and flat-spectrum radio-quasars. It is shown that there is a statistically significant difference between the kinematics of the two AGN classes, with BL Lacs showing more bent jets, that are wider and show slower movement along the jet axis, compared to flat-spectrum radio-quasars. This is interpreted as evidence for helically structured jets (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Context. Recent VLBI studies of the morphology and kinematics of individual BL Lac objects (S5 1803+784, PKS 0735+178, etc.) have revealed a new paradigm for the pc-scale jet kinematics of these sources. Unlike the apparent superluminal outward motions usually observed in blazars, most, if not all, jet components in these sources appear to be stationary with respect to the core, while exhibiting strong changes in their position angles. As a result, the jet ridge lines of these sources evolve substantially, at times forming a wide channel-flow. Aims. We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample of radio-loud active galaxies to study this new kinematic scenario for flat-spectrum AGN. Comparing BL Lac objects and quasars in the CJF, we look for differences in the kinematics and morphologies of their jet ridge lines. The large number of sources in the CJF sample, together with the excellent kinematical data available, allow us to perform a robust statistical analysis in this context. Methods. We develop a number of tools to extract information about the apparent linear and angular evolution of the CJF jet ridge lines, as well as their morphology. We study both the radial and non-radial apparent motions in the CJF jets. A statistical analysis of the extracted information allows us to test this new kinematic scenario and assess the importances of the radial and non-radial motions in flat-spectrum AGN jets in general and especially in BL Lacs compared to quasars. We also use these tools to check the kinematics for (multi-wavelength) variable AGN. Results. We find that approximately half of the sample display appreciable apparent jet widths (>10 degrees), with BL Lac jet ridge lines showing significantly larger apparent widths than those of both quasars and radio galaxies. In addition, BL Lac jet ridge lines are found to change their apparent width more strongly. Finally, BL Lac jet ridge lines show the least apparent linear evolution, which translates into the smallest apparent expansion speeds for their components. We find compelling evidence supporting a substantially different kinematic scenario for flat-spectrum radio-AGN jets and in particular for BL Lac objects. In addition, we find that variability is closely related to the properties of a source's jet ridge line. Variable quasars are found to have "BL Lac like" behavior, relative to their non-variable counterparts [ABSTRACT FROM AUTHOR]

We present the result of our extensive intranight optical monitoring of the well-known low-energy peaked BL Lac (LBL) object PKS 0735+178. This long-term follow-up consists of R-band monitoring for a minimum duration of ∼4 hours, on 17 nights spanning 11 years (1998–2008). Using the CCD as an N-star photometer, a detection limit of around 1 per cent was attained for the intranight optical variability (INOV). Remarkably, an INOV amplitude of on hour-like time-scale was not observed on any of the 17 nights, even though the likelihood of a typical LBL showing such INOV levels in a single session of hours duration is known to be high . Our observations have thus established a peculiar long-term INOV quiescence of this radio-selected BL Lac object. Moreover, the access to unpublished optical monitoring data of similarly high sensitivity, acquired in another programme, has allowed us to confirm the same anomalous INOV quiescence of this LBL all the way back to 1989, the epoch of its historically largest radio outburst. Here, we present observational evidence revealing the very unusual INOV behaviour of this classical BL Lac object and discuss this briefly in the context of its other known exceptional properties. [ABSTRACT FROM AUTHOR]

The BL Lacertae object S5 1803+784 has been monitored with very long baseline interferometry at 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 8–11 c. 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. [ABSTRACT FROM AUTHOR]