A. P. Lobanov

Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. IV. Fine-Scale Structure

We have examined the compact structure in 250 flat-spectrum extragalactic radio sources using interferometric fringe visibilities obtained with the Very Long Baseline Array (VLBA) at 15 GHz. With projected baselines out to 440 Mλ, we are able to investigate source structure on typical angular scales as small as 0.05 mas. This scale is similar to the resolution of the VLBI Space Observatory Programme data obtained on longer baselines at a lower frequency and with somewhat poorer accuracy. For 171 sources in our sample, more than half of the total flux density seen by the VLBA remains unresolved on the longest baselines. There are 163 sources in our list with a median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest baselines; these will be useful as fringe finders for short-wavelength VLBA observations. The total flux densities recovered in the VLBA images at 15 GHz are generally close to the values measured around the same epoch at the same frequency with the RATAN-600 and University of Michigan Radio Astronomy Observatory telescopes. We have modeled the core of each source with an elliptical Gaussian component. For about 60% of the sources, we have at least one observation in which the core component appears unresolved (generally smaller than 0.05 mas) in one direction, usually transverse to the direction into which the jet extends. BL Lac objects are on average more compact than quasars, while active galaxies are on average less compact. Also, in an active galaxy the sub-milliarcsecond core component tends to be less dominant. Intraday variability (IDV) sources typically have a more compact, more core-dominated structure on sub-milliarcsecond scales than non-IDV sources, and sources with a greater amplitude of intraday variations tend to have a greater unresolved VLBA flux density. The objects known to be GeV gamma-ray-loud appear to have a more compact VLBA structure than the other sources in our sample. This suggests that the mechanisms for the production of gamma-ray emission and for the generation of compact radio synchrotron–emitting features are related. The brightness temperature estimates and lower limits for the cores in our sample typically range between 1011 and 1013 K, but they extend up to 5 × 1013 K, apparently in excess of the equipartition brightness temperature or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed in radio sources with high observed brightness temperatures, as would be expected from relativistic beaming. Longer baselines, which can be obtained by space VLBI observations, will be needed to resolve the most compact high brightness temperature regions in these sources.

MOJAVE: Monitoring of Jets in Active galactic nuclei with VLBA Experiments - IX. Nuclear opacity

Aims. We have investigated a frequency-dependent shift in the absolute position of the optically thick apparent origin of parsec-scale jets (“core shift” effect) to probe physical conditions in ultra-compact relativistic outflows in active galactic nuclei. Methods. We used multi-frequency Very Long Baseline Array (VLBA) observations of 191 sources carried out in 12 epochs in 2006 within the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) program. The observations were performed at 8.1, 8.4, 12.1, and 15.4 GHz. We implemented a method of determining the core shift vector based on (i) image registration by two-dimensional normalized cross-correlation and (ii) model-fitting the source brightness distribution to take into account a non-zero core component offset from the phase center. Results. The 15.4−8.1, 15.4−8.4, and 15.4−12.1 GHz core shift vectors are derived for 163 sources, and have median values of 128, 125, and 88 μas, respectively, compared to the typical measured errors of 50, 51, 35 μas. The effect occurs predominantly along the jet direction, with departures smaller than 45° from the median jet position angle in over 80% of the cases. Despite the moderate ratio of the observed frequencies ( 2σ) are detected for about 55% of the sources. These shifts are even better aligned with the jet direction, deviating from the latter by less than 30° in over 90% of the cases. There is an indication that the core shift decreases with increasing redshift. Magnetic fields in the jet at a distance of 1 parsec from the central black hole, calculated from the obtained core shifts, are found to be systematically stronger in quasars (median B_1 ≈ 0.9 G) than those in BL Lacs (median B_1 ≈ 0.4 G). We also constrained the absolute distance of the core from the apex of the jet at 15 GHz as well as the magnetic field strength in the 15 GHz core region.

Intraday variability in compact extragalactic radio sources - II. Observations with the Effelsberg 100 m radio telescope

We present the data from 11 observing campaigns (carried out between 1989 and 1999) at the Effelsberg 100 m radio telescope to study Intraday Variability in Active Galactic Nuclei. Most of these observations were performed in total power and linear polarization. We give summary tables, light curves, and structure functions from these data sets. Due to the large number of individual observations, only examples of the lightcurves will be presented here; the complete set of figures will be accessible online �� . Intraday variations are present in nearly all sources (detected during at least one of the observing campaigns). Variations in total flux density are usually accompanied by similar variability of the linear polarization. In most cases, the latter variations are stronger and faster by up to a factor of two.

Spectral Evolution of the Parsec-Scale Jet in the Quasar 3C 345

The long-term evolution of the synchrotron emission from the parsec-scale jet in the quasar 3C345 is analysed, on the basis of multi-frequency monitoring with very long baseline interferometry (VLBI) and covering the period 1979-1994. We combine the model fits from 44 VLBI observations of 3C345 made at 8 different frequencies between 2.3 and 100GHz. We calculate the turnover frequency, turnover flux density, integrated 4-25GHz flux and 4-25GHz luminosity of the core and the moving features. The core has an estimated mean luminosity of 7.1+/-3.5 * 10^42 erg/s; the estimated total luminosity of 3C345 on parsec scales is ~3*10^43 erg/s (about 1% of the observed luminosity of the source between the radio to infrared regimes). The spectral changes in the core can be reconciled with a shock or dense plasma condensation traveling through the region where the jet becomes optically thin. We are able to describe the evolution of the core spectrum by a sequence of 5 flare-like events characterized by an exponential rise and decay of the particle number density of the material injected into the jet. The same model is also capable of predicting the changes in the flux density observed in the core. The flares occur approximately every 3.5-4 years, roughly correlating with appearances of new moving features in the jet.The long-term evolution of the synchrotron emission from the parsec-scale jet in the quasar 3C 345 is analyzed on the basis of multifrequency monitoring with very long baseline interferometry (VLBI) and covering the period 1979-1994. We demonstrate that the compact radio structure of 3C 345 can be adequately represented by Gaussian model fits and that the model fits at different frequencies are sufficiently reliable for studying the spectral properties of the jet. We combine the model fits from 44 VLBI observations of 3C 345 made at eight different frequencies between 2.3 and 100 GHz. This combined database is used for deriving the basic properties of the synchrotron spectra of the VLBI core and the moving features observed in the jet. We calculate the turnover frequency, the turnover flux density, and the integrated 4-25 GHz flux and 4-25 GHz luminosity of the core and the moving features. The core has an estimated mean luminosity Lcore = (7.1 ± 3.5) × 1042 h-2 ergs s-1; the estimated total luminosity of 3C 345 on parsec scales is ≈3 × 1043 h-2 ergs s-1 (about 1% of the observed luminosity of the source between the radio to infrared regimes). The luminosities of the core and most of the moving features decrease at the average rate of 1.2 × 1035 h-2 (0.74 ± 0.06)t-1979.0 ergs s-2 (t measured in years). The derived luminosity variations require intrinsic acceleration of the moving features. The turnover frequency of one of the moving features reaches a peak during the above period. The combination of the overall spectral and kinematic changes in that feature cannot be reproduced satisfactorily by relativistic shocks, which may indicate rapid dissipation in shocks. The spectral changes in the core can be reconciled with a shock or dense plasma condensation traveling through the region where the jet becomes optically thin. We are able to describe the evolution of the core spectrum by a sequence of five flarelike events characterized by an exponential rise and decay of the particle number density of the material injected into the jet. The same model is also capable of predicting the changes in the flux density observed in the core. The flares occur approximately every 3.5-4 yr, roughly correlating with appearances of new moving features in the jet and indicating that a quasi-periodic process in the nucleus may be driving the observed emission and structural evolution of 3C 345.

Observational evidence for the link between the variable optical continuum and the subparsec-scale jet of the radio galaxy 3C 390.3

The mechanism and the region of generation of variable continuum emission are poorly understood for radio-loud active galactic nuclei because of the complexity of the nuclear region. High-resolution radio very long baseline interferometry (VLBI) observations have allowed us to zoom into a subparsec-scale region of the jet in the radio-loud galaxy 3C 390.3. We have combined the radio VLBI and the optical data covering the time period of 14 yr in order to look for a link between the optical flares and the parsec-scale jet. We have identified two stationary and nine moving radio features in the innermost subparsec-scale region of the jet. All nine ejections are associated with optical flares. We have found a significant correlation (at a confidence level of >99.99 per cent) between the ejected jet components and the optical continuum flares. The epochs at which the moving knots pass through the location of a stationary radio feature and at which the optical light curve reaches maximum are correlated. The radio events follow the maxima of optical flares with a mean time delay of 0.10 ± 0.04 yr. This correlation can be understood if the variable optical continuum emission is generated in the innermost part of the jet. A possible mechanism of the energy release is the ejection of knots of high-energy electrons that are accelerated in the jet flow and generate flares of synchrotron continuum emission in the wide range of frequencies from radio to X-ray bands. In this scenario, the beamed optical continuum emission from the jet and counterjet ionizes a gas in a subrelativistic outflow surrounding the jet. This results in the formation of two outflowing conical regions with broad emission lines (in addition to the conventional broad-line region around the central nucleus).

FLARE-LIKE VARIABILITY OF THE Mg II λ2800 EMISSION LINE IN THE γ-RAY BLAZAR 3C 454.3

We report the detection of a statistically significant flare-like event in the Mg II λ2800 emission line of 3C 454.3 during the outburst of autumn 2010. The highest levels of emission line flux recorded over the monitoring period (2008-2011) coincide with a superluminal jet component traversing through the radio core. This finding crucially links the broad emission line fluctuations to the non-thermal continuum emission produced by relativistically moving material in the jet and hence to the presence of broad-line region clouds surrounding the radio core. If the radio core were located at several parsecs from the central black hole, then our results would suggest the presence of broad-line region material outside the inner parsec where the canonical broad-line region is envisaged to be located. We briefly discuss the implications of broad emission line material ionized by non-thermal continuum in the context of virial black hole mass estimates and gamma-ray production mechanisms.

Relativistic Plasma as the Dominant Source of the Optical Continuum Emission in the Broad-Line Radio Galaxy 3C 120

We report a relation between radio emission in the inner jet of the Seyfert galaxy 3C 120 and optical continuum emission in this galaxy. Combining the optical variability data with multi-epoch high-resolution very long baseline interferometry observations reveals that an optical flare rises when a superluminal component emerges into the jet and its maxima is related to the passage of such component through the location a stationary feature at a distance of ~1.3 parsecs from the jet origin. This indicates that a significant fraction of the optical continuum produced in 3C 120 is non-thermal and it can ionize material in a sub-relativistic wind or outflow. We discuss implications of this finding for the ionization and structure of the broad emission line region, as well as for the use of broad emission lines for determining black hole masses in radio-loud AGN.

The powerful jet of an off-nuclear intermediate-mass black hole in the spiral galaxy NGC 2276

Jet ejection by accreting black holes is a mass invariant mechanism unifying stellar and supermassive black holes (SMBHs) that should also apply for intermediate-mass black holes (IMBHs),whicharethoughttobetheseedsfromwhichSMBHsform.Wepresentthedetection of an off-nuclear IMBH of ∼5 × 10 4 Mlocated in an unusual spiral arm of the galaxy NGC 2276 based on quasi-simultaneous Chandra X-ray observations and European VLBI Network (EVN) radio observations. The IMBH, NGC2276-3c, possesses a 1.8 pc radio jet that is oriented in the same direction as large-scale (∼650 pc) radio lobes and whose emission is consistent with flat to optically thin synchrotron emission between 1.6 and 5 GHz. Its jet kinetic power (4 × 10 40 erg s −1 ) is comparable to its radiative output and its jet efficiency (≥46percent) is as large as that of SMBHs. A region of ∼300 pc along the jet devoid of young stars could provide observational evidence of jet feedback from an IMBH. The discovery confirms that the accretion physics is mass invariant and that seed IMBHs in the early Universe possibly had powerful jets that were an important source of feedback.

Relativistic outflow drives γ-ray emission in 3C 345

Aims: 3C345 was recently identified as a gamma-ray emitter, based on the first 20 months of Fermi-LAT data and optical monitoring. In this paper, a connection between the gamma-ray and optical variability of 3C345 and properties of its parsec-scale radio emission is investigated. Methods: The Fermi-LAT data of 3C345, covering an energy range of 0.1-300 GeV, were combined with 32 Very Long Baseline Array observations of the object made at 43.2 GHz in the period of January 2008 - March 2010. Results: The VLBA data reveal morphology and kinematics of the flow on scales of up to ~5 milliarcseconds (mas; deprojected linear distances of 380 parsecs). The brightness temperature, T_b(r), measured along the jet first decreases with distance proportional to r^-(0.95 +/-0.69) and later exhibits a break at ~0.3 mas, with T_b(r) proportional to r^-(4.11 +/-0.85) at larger separations. Variations of the gamma-ray, optical and parsec-scale radio emission show a similar long-term trend persistent during the entire VLBA monitoring period. The gamma-ray and optical variations on shorter time scales are related to structural changes in the jet on scales of ~0.3 mas (~23 parsecs, deprojected), with the gamma-ray and optical flares possibly related to the evolution of four distinct superluminal components identified in the flow. Conclusions: The observations indicate that both the quiescent and flaring components of the gamma-ray emission are produced in a region of the jet of ~23 pc in extent. This region may mark the Compton-loss dominated zone of the flow and its large extent may favor the synchrotron self-Compton mechanism for gamma-ray production in the relativistic jet of the quasar 3C345.

Catching the radio flare in CTA 102 - III. Core-shift and spectral analysis

The temporal and spatial spectral evolution of the jets of AGN can be studied with multi-frequency, multi-epoch VLBI observations. The combination of both, morphological and spectral parameters can be used to derive source intrinsic physical properties such as the magnetic field and the non-thermal particle density. In the first two papers of this series, we analyzed the single-dish light curves and the VLBI kinematics of the blazar CTA 102 and suggested a shock-shock interaction between a traveling and a standing shock wave as a possible scenario to explain the observed evolution of the component associated to the 2006 flare. In this paper we investigate the core-shift and spectral evolution to test our hypothesis of a shock-shock interaction. We used 8 multi-frequency VLBA observations to analyze the temporal and spatial evolution of the spectral parameters during the flare. We observed CTA 102 between May 2005 and April 2007 using the VLBA at six different frequencies spanning from 2 - 86 GHz. After the calibrated VLBA images were corrected for opacity, we performed a detailed spectral analysis. From the derived values we estimated the magnetic field and the density of the relativistic particles. The detailed analysis of the opacity shift reveals that the position of the jet core is proportional to nu^-1 with some temporal variations. The value suggests possible equipartition between magnetic field energy and particle kinetic energy densities at the most compact regions. From the variation of the physical parameters we deduced that the 2006 flare in CTA 102 is connected to the ejection of a new traveling feature (t=2005.9) and the interaction between this shock wave and a stationary structure around 0.1 mas from the core. The source kinematics together with the spectral and structural variations can be described by helical motions in an over-pressured jet.