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Context. 3C 84 is a nearby radio source with a complex total intensity structure, showing linear polarisation and spectral patterns. A detailed investigation of the central engine region necessitates the use of very-long-baseline interferometry (VLBI) above the hitherto available maximum frequency of 86 GHz. Aims. Using ultrahigh resolution VLBI observations at the currently highest available frequency of 228 GHz, we aim to perform a direct detection of compact structures and understand the physical conditions in the compact region of 3C 84. Methods. We used Event Horizon Telescope (EHT) 228 GHz observations and, given the limited (u, v)-coverage, applied geometric model fitting to the data. Furthermore, we employed quasi-simultaneously observed, ancillary multi-frequency VLBI data for the source in order to carry out a comprehensive analysis of the core structure. Results. We report the detection of a highly ordered, strong magnetic field around the central, supermassive black hole of 3C 84. The brightness temperature analysis suggests that the system is in equipartition. We also determined a turnover frequency of νm = (113 ± 4) GHz, a corresponding synchrotron self-absorbed magnetic field of BSSA = (2.9 ± 1.6) G, and an equipartition magnetic field of Beq = (5.2 ± 0.6) G. Three components are resolved with the highest fractional polarisation detected for this object (mnet = (17.0 ± 3.9)%). The positions of the components are compatible with those seen in low-frequency VLBI observations since 2017–2018. We report a steeply negative slope of the spectrum at 228 GHz. We used these findings to test existing models of jet formation, propagation, and Faraday rotation in 3C 84. Conclusions. The findings of our investigation into different flow geometries and black hole spins support an advection-dominated accretion flow in a magnetically arrested state around a rapidly rotating supermassive black hole as a model of the jet-launching system in the core of 3C 84. However, systematic uncertainties due to the limited (u, v)-coverage, however, cannot be ignored. Our upcoming work using new EHT data, which offer full imaging capabilities, will shed more light on the compact region of 3C 84. [ABSTRACT FROM AUTHOR]

Context. Flares in radio-loud active galactic nuclei are thought to be associated with the injection of fresh plasma into the compact jet base. Such flares are usually strongest and appear earlier at shorter radio wavelengths. Hence, very long baseline interferometry (VLBI) at millimeter(mm)-wavelengths is the best-suited technique for studying the earliest structural changes of compact jets associated with emission flares. Aims. We study the morphological changes of the parsec-scale jet in the nearby (z = 0.049) γ-ray bright radio galaxy 3C 111 following a flare that developed into a major radio outburst in 2007. Methods. We analyse three successive observations of 3C 111 at 86 GHz with the Global mm-VLBI Array (GMVA) between 2007 and 2008 which yield a very high angular resolution of ∼45 μas. In addition, we make use of single-dish radio flux density measurements from the F-GAMMA and POLAMI programmes, archival single-dish and VLBI data. Results. We resolve the flare into multiple plasma components with a distinct morphology resembling a bend in an otherwise remarkably straight jet. The flare-associated features move with apparent velocities of ∼4.0c to ∼4.5c and can be traced also at lower frequencies in later epochs. Near the base of the jet, we find two bright features with high brightness temperatures up to ∼1011 K, which we associate with the core and a stationary feature in the jet. Conclusions. The flare led to multiple new jet components indicative of a dynamic modulation during the ejection. We interpret the bend-like feature as a direct result of the outburst which makes it possible to trace the transverse structure of the jet. In this scenario, the components follow different paths in the jet stream consistent with expectations for a spine-sheath structure, which is not seen during intermediate levels of activity. The possibility of coordinated multiwavelength observations during a future bright radio flare in 3C 111 makes this source an excellent target for probing the radio-γ-ray connection. [ABSTRACT FROM AUTHOR]

Context. The advent of the Fermi gamma-ray space telescope with its superb sensitivity, energy range, and unprecedented capability to monitor the entire 4π sky within less than 2–3 h, introduced a new standard in time domain gamma-ray astronomy. Among several breakthroughs, Fermi has – for the first time – made it possible to investigate, with high cadence, the variability of the broadband spectral energy distribution (SED), especially for active galactic nuclei (AGN). This is necessary for understanding the emission and variability mechanisms in such systems. To explore this new avenue of extragalactic physics the Fermi-GST AGN Multi-frequency Monitoring Alliance (F-GAMMA) programme undertook the task of conducting nearly monthly, broadband radio monitoring of selected blazars, which is the dominant population of the extragalactic gamma-ray sky, from January 2007 to January 2015. In this work we release all the multi-frequency light curves from 2.64 to 43 GHz and first order derivative data products after all necessary post-measurement corrections and quality checks. Aims. Along with the demanding task to provide the radio part of the broadband SED in monthly intervals, the F-GAMMA programme was also driven by a series of well-defined fundamental questions immediately relevant to blazar physics. On the basis of the monthly sampled radio SEDs, the F-GAMMA aimed at quantifying and understanding the possible multiband correlation and multi-frequency radio variability, spectral evolution and the associated emission, absorption and variability mechanisms. The location of the gamma-ray production site and the correspondence of structural evolution to radio variability have been among the fundamental aims of the programme. Finally, the programme sought to explore the characteristics and dynamics of the multi-frequency radio linear and circular polarisation. Methods. The F-GAMMA ran two main and tightly coordinated observing programmes. The Effelsberg 100 m telescope programme monitoring 2.64, 4.85, 8.35, 10.45, 14.6, 23.05, 32, and 43 GHz, and the IRAM 30 m telescope programme observing at 86.2, 142.3, and 228.9 GHz. The nominal cadence was one month for a total of roughly 60 blazars and targets of opportunity. In a less regular manner the F-GAMMA programme also ran an occasional monitoring with the APEX 12 m telescope at 345 GHz. We only present the Effelsberg dataset in this paper. The higher frequencies data are released elsewhere. Results. The current release includes 155 sources that have been observed at least once by the F-GAMMA programme. That is, the initial sample, the revised sample after the first Fermi release, targets of opportunity, and sources observed in collaboration with a monitoring programme following up on Planck satellite observations. For all these sources we release all the quality-checked Effelsberg multi-frequency light curves. The suite of post-measurement corrections and flagging and a thorough system diagnostic study and error analysis is discussed as an assessment of the data reliability. We also release data products such as flux density moments and spectral indices. The effective cadence after the quality flagging is around one radio SED every 1.3 months. The coherence of each radio SED is around 40 min. Conclusions. The released dataset includes more than 3 × 104 measurements for some 155 sources over a broad range of frequencies from 2.64 GHz to 43 GHz obtained between 2007 and 2015. The median fractional error at the lowest frequencies (2.64–10.45 GHz) is below 2%. At the highest frequencies (14.6–43 GHz) with limiting factor of the atmospheric conditions, the errors range from 3% to 9%, respectively. [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]

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]

Aims. We report about a 230 GHz very long baseline interferometry (VLBI) fringe finder observation of blazar 3C 279 with the APEX telescope in Chile, the phased submillimeter array (SMA), and the SMT of the Arizona Radio Observatory (ARO). Methods. We installed VLBI equipment and measured the APEX station position to 1 cm accuracy (1ix). We then observed 3C 279 on 2012 May 7 in a 5 h 230 GHz VLBI track with baseline lengths of 2800 Md to 7200 Md and a finest fringe spacing of 28.6uas. Results. Fringes were detected on all baselines with signal-to-noise ratios of 12 to 55 in 420 s. The correlated flux density on the longest baseline was ~0.3 Jy beam-1, out of a total flux density of 19.8 Jy. Visibility data suggest an emission region <38uas in size, and at least two components, possibly polarized. We find a lower limit of the brightness temperature of the inner jet region of about 1010 K. Lastly, we find an upper limit of 20% on the linear polarization fraction at a fringe spacing of ~38uas. Conclusions. With APEX the angular resolution of 230 GHz VLBI improves to 28.6uas. This allows one to resolve the last-photon ring around the Galactic Center black hole event horizon, expected to be 40uas in diameter, and probe radio jet launching at unprecedented resolution, down to a few gravitational radii in galaxies like M 87. To probe the structure in the inner parsecs of 3C 279 in detail, follow-up observations with APEX and five other mm-VLBI stations have been conducted (March 2013) and are being analyzed. [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]

Aims. The quasar S4 0917+624 is one of the targets in the Urumqi flux density monitoring program, which aims to study the properties of intra-day variable (IDV) extragalactic radio sources. Methods. The IDV observations of S4 0917+624 were carried out monthly, from August 2005 to January 2010, with the Urumqi 25 m radio telescope at 4.8 GHz. We analyze these and previous IDV observations to investigate the long-term IDV characteristics of S4 0917+624. We also study the long-term structural variability on milliarcsecond scales using VLBI maps obtained at 15 GHz (taken from the MOJAVE database) in order to search for a possible relation between variations in the IDV pattern and the source structure. Results. The quasar S4 0917+624 exhibits only very weak or no IDV during our 4.5 year observing interval. Prior to the year 2000, the source S4 0917+624 was one of the most prominent IDV sources. Our new data indicate that the previous strong IDV has ceased. We analyzed the long-term VLBI structural variability using Gaussian model-fitting. From this we obtained the flux densities and the deconvolved sizes of core and inner-jet components of the source. We studied the properties such as core fraction, angular size, spectral index, and brightness temperature of VLBI core for S4 0917+624, as well as the time delay between 5 and 15 GHz variations, and compared them with the IDV properties of S4 0917+624. The source shows ejection of several jet components that are suspected to have partially reduced the IDV amplitude of S4 0917+624. However, during 2005-2006, the VLBI core size was comparable to the size before the year 2000, but no strong IDV was detected in the period, suggesting that the quenching effect due to source size changes may not be responsible for the lack of strong IDV after the year 2000. The refractive scattering properties for the strong IDV phase of S4 0917+624 before the year 2000 are discussed. Conclusions. The disappearance of strong IDV in S4 0917+624 after the year 2000 is a mystery and cannot be explained via the quenching effect by changes in the observable VLBI structure. However, it may be caused by changes in the interstellar medium, i.e. by interstellar weather, which induces changes in the scintillation pattern on timescales of several years. Further coordinated multifrequency observations will be required to distinguish between the effect of source-intrinsic variability and changing properties of the interstellar medium. [ABSTRACT FROM AUTHOR]

Context. With the current study we aim at understanding the properties of radio emission and the assumed jet from four radio-loud and γ-ray-loud narrow-line Seyfert 1 galaxies that have been detected by Fermi. These are Seyfert 1 galaxies with emission lines at the low end of the FWHM distribution. Aims. The ultimate goal is twofold: first we investigate whether a relativistic jet is operating at the source producing the radio output, and second, we quantify the jet characteristics to understand possible similarities with and differences from the jets found in typical blazars. Methods. We relied on the most systematic monitoring of radio-loud and γ-ray-detected narrow-line Seyfert 1 galaxies in the cm and mm radio bands conducted with the Effelsberg 100 m and IRAM 30 m telescopes. It covers the longest time-baselines and the most radio frequencies to date. This dataset of multi-wavelength, long-term radio light-curves was analysed from several perspectives. We developed a novel algorithm to extract sensible variability parameters (mainly amplitudes and time scales) that were then used to compute variability brightness temperatures and the corresponding Doppler factors. The jet powers were computed from the light curves to estimate the energy output and compare it with that of typical blazars. The dynamics of radio spectral energy distributions were examined to understand the mechanism causing the variability. Results. The length of the available light curves for three of the four sources in the sample allowed a firm understanding of the general behaviour of the sources. They all display intensive variability that appears to be occurring at a pace rather faster than what is commonly seen in blazars. The flaring events become progressively more prominent as the frequency increases and show intensive spectral evolution that is indicative of shock evolution. The variability brightness temperatures and the associated Doppler factors are moderate, implying a mildly relativistic jet. The computed jet powers show very energetic flows. The radio polarisation in one case clearly implies a quiescent jet underlying recursive flaring activity. Finally, in one case, the sudden disappearance of a γ-ray flare below some critical frequency in our band needs a more detailed investigation of the possible mechanism causing the evolution of broadband events. Conclusions. Despite the generally lower flux densities, the sources appear to show all typical characteristics seen in blazars that are powered by relativistic jets, such as intensive variability, spectral evolution across the different bands following evolutionary paths explained by travelling shocks, and Doppler factors indicating mildly relativistic speeds. [ABSTRACT FROM AUTHOR]

Using millimeter very long baseline interferometry (VLBI) observations of the BL Lac object S5 0716+714 from August 2008 to September 2013, we investigate variations in the core flux density and orientation of the sub-parsec scale jet, i.e. position angle. The γ-ray data obtained by the Fermi Large Area Telescope are used to investigate the high-energy flux variations over the same time period. For the first time in any blazar, we report a significant correlation between the γ-ray flux variations and the position angle variations in the VLBI jet. The cross-correlation analysis also indicates a positive correlation such that the mm-VLBI core flux density variations are delayed with respect to the γ-ray flux by 82 ±32 days. This suggests that the high-energy emission is coming from a region located ±(3.8 ± 1.9) parsecs upstream of the mm-VLBI core (closer to the central black hole). These results imply that the observed inner jet morphology has a strong connection with the observed γ-ray flares. [ABSTRACT FROM AUTHOR]

Conte×t. This is the third paper in a series devoted to the analysis of the multiwavelength data from a campaign on the nearby (z = 0.03) TeV blazar Mrk 421 during 2011. Aims. We investigate the structure of the high angular resolution polarization, the magnetic topology, the total intensity light curve, the γ-ray flu×, and the photon inde×. We describe how they evolve and how they are connected. Methods. We analyzed data in polarized intensity obtained with the Very Long Baseline Array (VLBA) at twelve epochs (one observation per month from January to December 2011) at 15, 24, and 43 GHz. For the absolute orientation of the electric vector position angles (EVPA) we used the D-terms method; we also confirm its accuracy. We also used γ-ray data from the Fermi Large Area Telescope on weekly time bins throughout 2011. Results. The source shows polarized emission, and its properties vary with time, frequency, and location along the jet. The core mean polarization fraction is generally between 1% and 2%, with a 4% peak at 43 GHz in March; the polarization angle is variable, mainly at 15 GHz, where it changes frequently, and less so at 43 GHz, where it oscillates in the range 114°-173°∘. The jet polarization properties are more stable, with a fractional polarization of around 16% and a polarization angle nearly perpendicular to the jet a×is. The average flu× and photon inde× at γ-ray energies are (17.4 ± 0.5) ×10-8 ph cm-2 s-1 and Γ = 1.77 ± 0.02. The γ-ray light curve shows variability, with a main peak of (38 ± 11) × 10-8 ph cm-2 s-1 at the beginning of March and two later peaks centered on September 8 and November 13. The first γ-ray peak appears to be associated with the peak in the core polarized emission at 43 GHz, as well as with the total intensity light curve. A discrete correlation function analysis yields a correlation coefficient of 0.54 at zero delay, with a significance level >99.7%. Conclusions. With this multifrequency study, we accurately determine the polarization properties of Mrk 421, both in the core and in the jet region. The radio and γ-ray light curves are correlated. The observed EVPA variability at 15 GHz is partly due to opacity and partly to a variable Faraday rotation effect. To e×plain the residual variability of the intrinsic polarization angle and the low degree of polarization in the core region, we invoke a blend of variable cross-polarized subcomponents with different polarization properties within the beam. [ABSTRACT FROM AUTHOR]

Context. High-resolution radio observations allow us to directly image the innermost region of active galactic nuclei. The Very Long Baseline Array (VLBA) data analyzed in this paper were obtained during a multiwavelength (MWL) campaign, carried out in 2011, from radio to very high energy gamma rays, on the TeV blazar Markarian 421 (Mrk 421). Aims. Our aim was to obtain information on the jet structure in Mrk 421 during the MWL campaign at the highest possible angular resolution and with high temporal frequency observations, in order to compare structural and flux density evolution with higher energy variations. Methods. We consider data obtained with the VLBA at 43 GHz through two sets of observations: one is part of a dedicated multifrequency monitoring campaign, in which we observed Mrk 421 once a month from January to December 2011 at three frequencies; the other is extracted from the Boston University monitoring program, which observes 34 blazars at 43 GHz about once per month. We model-fit the data in the visibility plane, study the proper motion of jet components, the light curve, and the spectral index of the jet features. We compare the radio data with optical light curves obtained at the Steward Observatory, considering also the optical polarization information. Results. Mrk 421 has a bright nucleus and a one-sided jet extending towards the north-west for a few parsecs. The model-fits show that brightness distribution is well described using 6-7 circular Gaussian components, four of which are reliably identified at all epochs; all components are effectively stationary except for component D, at ∼0.4 mas from the core, whose motion is, however, subluminal. Analysis of the light curve shows two different states, with the source being brighter and more variable in the first half of 2011 than in the second half. The highest flux density is reached in February. A comparison with the optical data reveals an increase of the V magnitude and of the fractional polarization simultaneous with the enhancement of the radio activity. [ABSTRACT FROM AUTHOR]

We present a γ-ray photon flux and spectral variability study of the flat-spectrum radio quasar 3C 273 over a rapid flaring activity period between September 2009 to April 2010. Five major flares were observed in the source during this period. The most rapid flare observed in the source has a flux doubling time of 1.1 hr. The rapid γ-ray flares allow us to constrain the location and size of the γ-ray emission region in the source. The γ-opacity constrains the Doppler factor δγ ⩾ 10 for the highest energy (15 GeV) photon observed by the Fermi-Large Area Telescope (LAT). Causality arguments constrain the size of the emission region to 1.6 × 1015 cm. The γ-ray spectra measured over this period show clear deviations from a simple power law with a break in the 1-2 GeV energy range. We discuss possible explanations for the origin of the γ-ray spectral breaks. Our study suggests that the γ-ray emission region in 3C 273 is located within the broad line region (<1.6 pc). The spectral behavior and temporal characteristics of the individual flares indicate the presence of multiple shock scenarios at the base of the jet. [ABSTRACT FROM AUTHOR]

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]

We present the results of a series of radio, optical, X-ray, and γ-ray observations of the BL Lac object S50716+714 carried out between April 2007 and January 2011. The multifrequency observations were obtained using several ground- and space-based facilities. The intense optical monitoring of the source reveals faster repetitive variations superimposed on a long-term variability trend on a time scale of ~350 days. Episodes of fast variability recur on time scales of ~60-70 days. The intense and simultaneous activity at optical and γ-ray frequencies favors the synchrotron self-Compton mechanism for the production of the high-energy emission. Two major low-peaking radio flares were observed during this high optical/γ-ray activity period. The radio flares are characterized by a rising and a decaying stage and agrees with the formation of a shock and its evolution. We found that the evolution of the radio flares requires a geometrical variation in addition to intrinsic variations of the source. Different estimates yield robust and self-consistent lower limits of δ ⩾ 20 and equipartition magnetic field Beq ⩾ 0.36 G. Causality arguments constrain the size of emission region ϴ ⩽ 0.004 mas. We found a significant correlation between flux variations at radio frequencies with those at optical and γ-rays. The optical/GeV flux variations lead the radio variability by ~65 days. The longer time delays between low-peaking radio outbursts and optical flares imply that optical flares are the precursors of radio ones. An orphan X-ray flare challenges the simple, one-zone emission models, rendering them too simple. Here we also describe the spectral energy distribution modeling of the source from simultaneous data taken through different activity periods. [ABSTRACT FROM AUTHOR]

Context.PKS 1749+096 is a BL Lac object showing weak extended jet emission to the northeast of the compact VLBI core on parsec scales. Aims. We aim at better understanding the jet kinematics and variability of this source and finding clues that may applicable to other BL Lac objects. Methods. The jet was studied with multi-epoch multi-frequency high-resolution VLBI observations. Results. The jet is characterized by a one-sided curved morphology at all epochs and all frequencies. The VLBI core, located at the southern end of the jet, was identified based on its spectral properties. The equipartition magnetic field of the core was investigated, through which we derived a Doppler factor of 5, largely consistent with that derived from kinematics (component C5). The study of the detailed jet kinematics at 22 and 15 GHz, spanning a period of more than 10 years, indicates the possible existence of a bimodal distribution of the jet apparent speed. Ballistic and non-ballistic components are found to coexist in the jet. Superluminal motions in the range of 5-21?c were measured in 11 distinct components. We estimated the physical jet parameters with the minimum Lorentz factor of 10.2 and Doppler factors in the range of 10.2-20.4 (component C5). The coincidence in time of the component's ejection and flares supports the idea that, at least in PKS 1749+096, ejection of new jet components is connected with major outbursts in flux density. For the best-traced component (C5) we found that the flux density decays rapidly as it travels downstream the jet, accompanied by a steepening of its spectra, which argues in favor of a contribution of inverse Compton cooling. These properties make PKS 1749+096 a suitable target for an intensive monitoring to decipher the variability phenomenon of BL Lac objects. [ABSTRACT FROM AUTHOR]

Aims. We aim to search for evidence of annual modulation in the time scales of the BL Lac object S5 0716+714. Methods. The intra-day variability (IDV) observations were carried out monthly from 2005 to 2009, with the Urumqi 25 m radio telescope at 4.8 GHz. Results. The source has shown prominent IDV as well as long-term flux variations. The IDV time scale does show evidence in favor of an annual modulation, suggesting that the IDV of 0716+714 is dominated by interstellar scintillation. The source underwent a strong outburst phase between mid-2008 and mid-2009; a second intense flare was observed in late 2009, but no correlation between the total flux density and the IDV time scale is found, implying that the flaring state of the source does not have serious implications for the general characteristics of its intra-day variability. However, we find that the inner-jet position angle is changing throughout the years, which could result in an annual modulation noise in the anisotropic ISS model fit. There is also an indication that the lowest IDV amplitudes (rms in flux density) correspond to the slowest time scales of IDV, which would be consistent with an ISS origin of the IDV of 0716+714. [ABSTRACT FROM AUTHOR]

The BLLac object S4 0954+65 is one of the main targets of the Urumqi monitoring program targeting intraday variable (IDV) sources. Between August 2005 and December 2009, the source was included in 41 observing sessions, carried out at a frequency of 4.8 GHz. The time analysis of the collected light curves, performed by applying both a structure function analysis and a specifically developed wavelet-based algorithm, discovered an annual cycle in the variability timescales, suggesting that there is a fundamental contribution by interstellar scintillation to the IDV pattern of the source. The combined use of the two analysis methods also revealed that there was a dramatic change in the variability characteristics of the source between February and March 2008, at the starting time of a strong outburst phase. The analysis' results suggest that the flaring state of the source coincides with the appearance of multiple timescales in its light curves, indicating that changes in the structure of the relativistically moving emitting region may strongly influence the variability observed on IDV timescales. [ABSTRACT FROM AUTHOR]

An abstract of the article "On the calibration of full-polarization 86 GHz global VLBI observations," by I. Martí-Vidal, T. P. Krichbaum, A. Marscher, W. Alef, A. Bertarini, U. Bach, F. K. Schinzel, H. Rottmann, J. M. Anderson, J. A. Zensus, M. Bremer, and S. Sanchez is presented.