S. Britzen

Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71 III.Rapid and correlated flux density variability from radio to sub-mm bands

Aims. The BL Lac object S5 0716+71 was observed in a global multi-frequency campaign to search for rapid and correlated flux density variability and signatures of an inverse-Compton (IC) catastrophe during the states of extreme apparent brightness temperatures. Methods. The observing campaign involved simultaneous ground-based monitoring at radio to IR/optical wavelengths and was centered around a 500-ks pointing with the INTEGRAL satellite (November 10−17, 2003). Here, we present the combined analysis and results of the radio observations, covering the cm- to sub-mm bands. This facilitates a detailed study of the variability characteristics of an inter- to intra-day variable IDV source from cm- to the short mm-bands. We further aim to constrain the variability brightness temperatures (TB) and Doppler factors (δ) comparing the radio-bands with the hard X-ray emission, as seen by INTEGRAL at 3−200 keV. Results. 0716+714 was in an exceptionally high state and different (slower) phase of short-term variability, when compared to the past, most likely due to a pronounced outburst shortly before the campaign. The flux density variability in the cm- to mm-bands is dominated by a ∼ 4d ay time scale amplitude increase of up to ∼35%, systematically more pronounced towards shorter wavelengths. The cross-correlation analysis reveals systematic time-lags with the higher frequencies varying earlier, similar to canonical variability on longer time-scales. The increase of the variability amplitudes with frequency contradicts expectations from standard interstellar scintillation (ISS) and suggests a source-intrinsic origin for the observed inter-day variability. We find an inverted synchrotron spectrum peaking near 90 GHz, with the peak flux increasing during the first 4 days. The lower limits to TB derived from the inter-day variations exceed the 10 12 K IC-limit by up to 3−4 orders of magnitude. Assuming relativistic boosting, our different estimates of δ yield robust and self-consistent lower limits of δ ≥ 5−33 – in good agreement with δVLBI obtained from VLBI studies and the IC-Doppler factors δIC > 14−16 obtained from the INTEGRAL data. Conclusions. The non-detection of S5 0716+714 with INTEGRAL in this campaign excludes an excessively high X-ray flux associated with a simultaneous IC catastrophe. Since a strong contribution from ISS can be excluded, we conclude that relativistic Doppler boosting naturally explains the apparent violation of the theoretical limits. All derived Doppler factors are internally consistent, agree with the results from different observations and can be explained within the framework of standard synchrotron-self-Compton (SSC) jet models of AGN.

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.

Kinematic study of the blazar S5 0716+714

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

A multi-epoch VLBI survey of the kinematics of CFJ sources. II. Analysis of the kinematics

Context. This is the second of a series of papers presenting VLBI observations of the 293 Caltech-Jodrell Bank Flat-Spectrum (hereafter CJF) sources and their analysis. Aims. To obtain a consistent motion dataset large enough to allow the systematic properties of the population to be studied. Methods. We present the detailed kinematic analysis of the complete fl ux-density limited CJF survey. We computed 2-D kinematic models based on the optimal model-fitting parameters of mult i-epoch VLBA observations. This allows us to calculate not only radial, but also orthogonal motions, and thus to study curvature and acceleration. Statistical tests of the motions measured an d their reliability have been performed. A correlation analysis between the derived apparent motions, luminosities, spectral indices, an d core dominance and the resulting consequences is described. Results. With at least one velocity in each of 237 sources, this sample is much larger than any available before and allows a meaningful statistical investigation of apparent motions and any poss ible correlations with other parameters in AGN jets. The main results to emerge are as follows: - In general motions are not consistent with a single uniform velocity applicable to all components along a jet. - We find a slight trend towards a positive outward accele ration and also adduce some evidence for greater acceleration in the inner-most regions. - We find a lack of fast components at phys ical distances less than a few pc from the reference feature. - Only ∼4% of the components from galaxies and<2% of those from quasars undergo large bends i.e. within 15 ◦ of±90 ◦ . - The distribution of radial velocities shows a broad distribution of velociti es (apparent velocities up to 30 c). 15% of the best sampled jet components exhibit ”low” velocities which may need to be explained in a different manner to the ”fast” motions. - Some negative or ”backwards” superluminal motions are seen and in 15 cases (6%) these are definitely significant. - We find a strong correlation between t he 5 GHz luminosity and the apparent velocity. - The CJF galaxies, on average, show slower apparent jet component velocities than the quasars. - The mean velocity in the VLBA 2cm survey (Kellermann et al. 2004) is substantially higher than in the CJF survey ‐ the rati o could be roughly a factor of 1.5‐2. This supports the observed trend of increasing apparent velocity with increasing observing frequency. Conclusions. This AGN survey provides the basis for any statistical analysis of jet and jet-component properties.

Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71 II. A search for intra-day variability at millimetre wavelengths with the IRAM 30 m telescope

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 (15.0 ± 1.8)%. Inter-day variability in linear polarization at 86 GHz, with significance level > 95%; σP/� P� = 15% and σχ = 6 ◦ , was observed during the first four observing days. From the total flux density variations at the synchrotron turnover frequency (∼86 GHz) we compute an apparent brightness temperature T app B > 1.4 × 10 14 K at a redshift of 0.3, which exceeds by two orders of magnitude the inverse-Compton limit. A relativistic correction for T app B with a Doppler factor δ> 7.8 brings the observed brightness temperature down to the inverse Compton limit. A more accurate lower limit of δ> 14.0, 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.

The IDV source J 1128+5925, a new candidate for annual modulation?

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 fur 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).

The kinematics in the pc-scale jets of AGN - The case of S5 1803+784

We present a kinematic analysis of jet component motion in the VLBI jet of the BL Lac object S5 1803+784, which does not reveal long-term outward motion for most of the components. Understanding the complex kinematic phenomena can possibly provide insights into the differences between quasars and BL Lac objects. The blazar S5 1803+784 has been studied with VLBI at

The soft X-ray properties of AGN from the CJF sample: A correlation analysis between soft X-ray and VLBI properties

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On the origin of compact radio sources The binary black hole model applied to the gamma-bright quasar PKS 0420{014

=1.6, 2.3, 5, 8.4, and 15 GHz between 1993.88 and 2005.68 in 26 observing runs. We (re)analyzed the data and present Gaussian model-fits. We collected the already published kinematic information for this source from the literature and re-identified the components according to the new scenario presented in this paper. Altogether, 94 epochs of observations have been investigated. A careful study of the long-term kinematics reveals a new picture for component motion in S5 1803+784. In contrast to previously discussed motion scenarios, we find that the jet structure within 12 mas of the core can most easily be described by the coexistence of several bright jet features that remain on the long-term at roughly constant core separations (in addition to the already known {\it stationary} jet component

A possible jet precession in the periodic quasar B0605-085

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