We present the results of a multi-frequency study of the structural evolution of the VLBI jet in the BL Lac object 0716+714 over the last 10 years. We show VLBI images obtained at 5 GHz, 8.4 GHz, 15 ...GHz and 22 GHz. The milliarcsecond source structure is best described by a one-sided core-dominated jet of $\sim $10 mas length. Embedded jet components move superluminally with speeds ranging from $5\,c$ to $16\,c$ (assuming $z=0.3$). Such fast superluminal motion is not typical of BL Lac objects, however it is still in the range of jet speeds typically observed in quasars ($10\,c$ to $20\,c$). In 0716+714, younger components that were ejected more recently seem to move systematically slower than the older components. This and a systematic position angle variation of the inner (1 mas) portion of the VLBI jet suggests an at least partly geometric origin of the observed velocity variations. The observed rapid motion and the derived Lorentz factors are discussed with regard to the rapid Intra-Day Variability (IDV) and the γ-ray observations, from which very high Doppler factors are inferred.
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Context. We present the soft X-ray properties obtained in the ROSAT All-Sky survey and from pointed PSPC observations for the AGN in the complete flux-density limited Caltech-Jodrell Bank flat ...spectrum sample (hereafter CJF). CJF is a VLBI survey (VLBA observations at 5 GHz) of 293 AGN with detailed information on jet component motion. Aims. We investigate and discuss the soft X-ray properties of this AGN sample and examine the correlations between X-ray and VLBI properties, test beaming scenarios, and search for the discriminating properties between the sub-samples detected and not detected by ROSAT. Methods. Comparing the observed and the predicted X-ray fluxes by assuming an Inverse Compton (IC) origin for the observed X- rays, we compute the beaming or Doppler factor, \delta_{\rm IC}, for the CJF sources and compare it with the equipartition Doppler factor, \delta_{\rm EQ}. We further contrast the Doppler factors with other beaming indicators derived from the VLBI observations, such as the value of the expansion velocity, and the observed and intrinsic brightness temperature. We calculate two different core dominance parameters ( R) : the ratio of total VLBI flux to single-dish flux, R_{\rm V}, and the ratio of the VLBI core-component flux to single-dish flux, R_{\rm C}. In addition, we investigate the large- scale radio structure of the AGN and the difference between the pc- and kpc- scale structure (misalignment) with regard to the X-ray observations. Results. We find a nearly linear relation between X-ray and radio luminosities, and a similar but less stringent behaviour for the relation between optical and X-ray luminosities. The CJF-quasars show faster apparent motions and larger values of \delta_{\rm IC} than the radio galaxies do. The quasars detected by ROSAT have a different \beta_{\rm app}-redshift relationship compared to the non-detected ones. We find no significant difference in R between the quasars detected and not detected by ROSAT. We find evidence that R is larger for quasars and BL Lac objects than it is for radio galaxies, in accordance with unification scenarios. ROSAT- detected sources tend to reveal extended large-scale radio structures more often. Conclusions. We conclude that beaming alone cannot explain the observed dichotomy of ROSAT detection or non-detection and assume that the large-scale jet structure plays a decisive role.
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P1; The search for periodic behavior in Blazars has been an important subject,which is helpful for providing significant clues to the structure and physical processes of their central energy engine. ...A binary black hole system has recently been suggested for causing precession of relativistic jets and rotation of the ejection position angle of VLBI knots in superluminal sources. It has been suggested that in QSO 3C345, the ejection direction of the superluminal knots rotates due to the precession of the central engine and thus the ejection position angle of the successive knots shows a periodic behavior. Some authors argue for a period of precession being～5.6 yr (Abraham & Caproni), ～8-10 yr (Klare et al.) and～9.5 yr (Lobanov & Roland). Applying the helical model proposed by Qian et al. and selecting appropriate parameters to fit the initial trajectories (within 0.3 mas) of all the components (C4 to C10), we derive the relation between the ejection position angle of the components and their precession phase, and thus find a 6.9-year precession period (4.3 yr in the source frame), which can fit the ejection position angle of all these superluminal knots well. Since the VLBI observations have covered more than two precession periods, confirmation in one or more future periods would be important.In addition, we emphasize that the initial parts of the trajectories of these knots can be fitted by a common helical pattern (channel) through a precessing of its initial phase.This scenario (or helical precessing model) is different from the usual ballistic precessing model in which the individual superluminal knots move along straight-lines after ejection (Tateyama & Kingham).
ABSTRACT
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 ∼ 106M⊙, 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 ≳ 108–9M⊙) 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 ($M_{BH}\, \sim \, 10^3 \rm{-}10^6 M_{\odot }$).
AGN jets under the microscope: A divide? Karouzos, M.; Britzen, S.; Witzel, A. ...
Astronomische Nachrichten,
June 2012, Volume:
333, Issue:
5-6
Journal Article
We report on a densely time sampled polarimetric flux density monitoring of the BL Lac object S5 0716+71 at 86 GHz and 229 GHz. The source was observed with the IRAM 30 m telescope at Pico Veleta ...within a coordinated multi-frequency observing campaign, which was centred around a 500 ks INTEGRAL observation during November 10 to 16, 2003. The aim of this campaign was to search for signatures of inverse-Compton catastrophes through the observation of the broad-band variability of the source. At 86 GHz, S5 0716+71 showed no intra-day variability, but showed remarkable inter-day variability with a flux density increase of 34% during the first four observing days, which cannot be explained by source extrinsic causes. At this frequency, making use of a new calibration strategy, we reach a relative rms accuracy of the flux density measurements of 1.2%. Although the flux density variability at 229 GHz was consistent with that at 86 GHz, the larger measurement errors at 229 GHz do not allow us to detect, with high confidence, inter-day variations at this frequency. At 86 GHz, the linear polarization fraction of S5 0716+71 was unusually large rm {(}15.0\pm1.8\rm {) }\%. Inter-day variability in linear polarization at 86 GHz, with significance level ga \rm \%; sigma_{P}/\langle P\rangle=15% and sigma_{\chi}=6\circ}, was observed during the first four observing days. From the total flux density variations at the synchrotron turnover frequency ( delta > 7.8 brings the observed brightness temperature down to the inverse Compton limit. A more accurate lower limit of \rm }$--> delta > \rm }, consistent with previous estimates from VLBI observations, is obtained from the comparison of the 86 GHz synchrotron flux density and the upper limits for the synchrotron self-Compton flux density obtained from the INTEGRAL observations. The relativistic beaming of the emission by this high Doppler factor explains the non-detection of "catastrophic" inverse-Compton avalanches by INTEGRAL.
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Context.Short time-scale radio variations of compact extragalactic radio sources, known as IntraDay Variability (IDV), can be explained in at least some sources by a source-extrinsic effect, in which ...the variations are interpreted as scintillation of radio waves caused by the turbulent interstellar medium of the Milky Way. One of the most convincing observational arguments in favour of propagation-induced variability is the so-called “annual modulation” of the characteristic variability time-scale, which is due to the orbital motion of the Earth. So far there are only two sources known which show such a well-defined seasonal cycle, a few more sources with fewer data can be regarded as possible candidates for this effect. However, source-intrinsic effects, such as structural variations, can also cause the observed changes of the variability time-scale. Data for the new, recently discovered, and highly variable IDV source J 1128+5925 are presented. Aims.We study the frequency and time dependence of the IDV in this compact quasar. We measure the characteristic variability time-scale of the IDV throughout the year, and analyze whether the observed changes in the variability time-scale are consistent with annual modulation. Assuming a radio wave propagation effect as origin, we are able to constrain some physical properties (such as distance, scattering-strength, and possible anisotropy) of the “plasma” screen, which may cause the scintillation. Methods.We monitored the flux density variability of J 1128+5925 with dense time sampling between 2.7 and 10.45 GHz. We observed with the 100 m Effelsberg radio telescope of the Max-Planck-Institut für Radioastronomie (MPIfR) at 2.70 GHz, 4.85 GHz, and 10.45 GHz, as well as with the 25 m Urumqi radio telescope (China) at 4.85 GHz. From ten observing sessions, each of which lasted several days during the period between 2004–2006, we determine the variability characteristics and time-scales which we investigate in view of possible scintillation and annual modulation. Results.The observed pronounced changes of the variability time-scale of J 1128+5925 are modelled with an anisotropic annual modulation model. The observed frequency dependence of the variation is in good agreement with the prediction from interstellar scintillation. Adopting a simple model for the annual modulation model and also using the frequency dependence of the IDV, we derive a lower limit to the distance the scattering screen and an upper limit for the scintillating source size. The latter is found to be consistent with the measured core size from Very Long Baseline Interferometry (VLBI).
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Context. This is the first of a series of papers presenting VLBI observations of the 293 Caltech-Jodrell Bank Flat-Spectrum (hereafter CJF) sources and their analysis. Aims. One of the major goals of ...the CJF is to make a statistical study of the apparent velocities of the sources. Methods. We have conducted global VLBI and VLBA observations at 5 GHz since 1990, accumulating thirteen separate observing campaigns. Results. We present here an overview of the observations, give details of the data reduction and present the source parameters resulting from a model-fitting procedure. For every source at every observing epoch, an image is shown, built up by restoring the model-fitted components, convolved with the clean beam, into the residual image, which was made by Fourier transforming the visibility data after first subtracting the model-fitted components in the uv-plane. Overplotted we show symbols to represent the model components. Conclusions. We have produced VLBI images of all but 5 of the 293 sources in the complete CJF sample at several epochs and investigated the kinematics of 266 AGN.
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We report the detection of the first VLBI ring-structure around the core of an AGN - around the core of the quasar 3C454.3. This ring-structure starts being visible in VLBI maps around 1996. It ...expands with an apparent velocity between 0.11±0.01mas/yr and 0.18±0.01mas/yr and dominates the pc-scale structure for at least 14 years. This is the result of a re-analysis of 41 VLBA data sets at six different radio frequencies observed between 1995.57 and 2011.48. We observe a correlation between radio flaring, flux-density variability, a ring-structure and kinematic properties of the jet. Taken together, it is tempting to see a causal connection and to explain all of this geometrically. The kinematic changes as well as the changes in the flaring characteristics might be caused by a change of the angle to the line of sight towards the observer. This behaviour resembles our findings for 0735+178 - with 3C454.3 being the second AGN to reveal kinematic mode changes. These mode changes could be explained by the presence of a supermassive binary black hole. 3C454.3 had been modelled as a binary black hole before.
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