T. Beckert

Circular polarization of radio emission from relativistic jets

In inhomogeneous optically thick synchrotron sources a substantial part of the electron population at low energies can be hidden by self-absorption and overpowered by high energy electrons in optically thin emission. These invisible electrons produce Faraday rotation and conversion, leaving their footprints in the linear and circular polarized radiation of the source. An important factor is also the magnetic field structure, which can be characterized in most cases by a global magnetic field and a turbulent component. We present the basic radiative transfer coefficients for polarized synchrotron radiation and apply them to the standard jet model for relativistic radio jets. The model can successfully explain the unusual circular and linear polarization of the Galactic Centre radio source Sgr A* and its sibling M 81*. It also can account for the circular polarization found in jets of more luminous quasars and X-ray binaries. The high ratio of circular to linear polarization requires the presence of a significant fraction of hidden matter and low-energy electrons in these jets. The stable handedness of circular polarization requires stable global magnetic field components with non-vanishing magnetic flux along the jet, while the low degree of total polarization implies also a significant turbulent field. The most favoured magnetic field configuration is that of a helix, while a purely toroidal field is unable to produce significant circular polarization. If connected to the magnetosphere of the black hole, the circular polarization and the jet direction determine the magnetic poles of the system which is stable over long periods of time. This may also have implications for possible magnetic field configurations in accretion flows.

Diffraction-limited bispectrum speckle interferometry of the nuclear region of the Seyfert galaxy NGC 1068 in the H and K' bands

We present near-infrared bispectrum speckle interferometry studies of the nuclear region of the Seyfert 2 galaxy NGC 1068. A diffraction-limited K � -band image with 74 mas resolution and the first H-band image with 57 mas resolution were reconstructed from speckle interferograms obtained with the SAO 6 m telescope. The resolved structure consists of a compact core and an extended northern and south-eastern component. The compact core is resolved at all position angles and has a north-western, tail-shaped extension as well as a fainter, south-eastern extension. The K � -band FWHM diameter of this compact core is approximately 18 × 39 mas or 1.3 × 2.8 pc (FWHM of a single-component Gaussian fit; fit range 30-80% of the telescope cut-off frequency; the diameter errors are ±4 mas), and the position angle (PA) of the north-western extension is -16 ± 4 ◦ . If 40% of the flux from the compact Kcore is emission from a point source and 60% from a Gaussian intensity distribution, then a slightly larger FWHM of approximately 26 × 58 mas is obtained for the compact Kcomponent. In the H band, the FWHM diameter of the compact core is approximately 18 × 45 mas (±4 mas), and the PA is -18 ± 4 ◦ . The extended northern component (PA ∼ 0 ◦ ) has an elongated structure with a length of about 400 mas or 29 pc. The extended south-eastern component is fainter than the northern component. The K � -a ndH-band fluxes from the resolved compact core were measured to be 350 ± 90 mJy (i.e., K � ∼ 8.2 m ) and 70 ± 20 mJy (H ∼ 10.4 m ), respectively. The PA of -16 ± 4 ◦ of the compact 18 × 39 mas core is very similar to that of the western wall (PA ∼ -15 ◦ ) of the bright region of the ionization cone. This suggests that the H- and K � -band emission from the compact core is both thermal emission and scattered light from dust near the western wall of a low-density, conical cavity or from the innermost region of a parsec-scale dusty torus that is heated by the central source (the dust sublimation radius of NGC 1068 is approximately 0.1-1 pc). The northern extended 400 mas structure lies near the western wall of the ionization cone and coincides with the inner radio jet (PA ∼ 11 ◦ ). The large distance from the core suggests that the K � -band emission of the northern extended component is scattered light from the western cavity region and the radio jet region.

The dynamical state of a thick cloudy torus around an AGN

We present stationary models of obscuring dusty tori around active galactic nuclei. Vertical and radial structure are discussed separately based on a dynamical approach to a cloudy accretion flow. The vertical cloud distribution is described by a modified isothermal disk for arbitrary thickness of the torus with a wide funnel along the symmetry axis. The radial accretion flow is a consequence of cloud-cloud collisions. Accretion in the combined gravitational potential of central black hole and stellar cluster generates free energy, which is dissipated in collisions, and maintains the thickness of the torus. We focus on the largest clouds at the tidal shear limit, which will dominate the dynamics and the appearance of torus. Successful obscuration as required by unified schemes of AGN is achieved for large mass accretion rates above the Eddington limit of the central black hole. A complete quantitative treatment for the putative torus in the prototypical Seyfert 2 nucleus of NGC 1068 is given.

Probing the dusty environment of the Seyfert 1 nucleus in NGC 3783 with MIDI/VLTI interferometry

Aims. We present mid-IR spectro-interferometry of the Seyfert type 1 nucleus of NGC 3783. The dusty circumnuclear environment is spatially resolved and the wavelength dependence of the compact emission is discussed. Methods. The observations were carried out with the MIDI instrument at the Very Large Telescope Interferometer in the N-band. Spectra and visibilities were derived with a spectral resolution of λ/Δλ ∼ 30 in the wavelength range from 8 to 13 μm. For the interpretation we developed a simple dusty disk model with a small and variable covering factor. Results. At baselines of 65 and 69 m, visibilities in the range of 0.4 to 0.7 were measured. The N-band spectra show a monotonic increase of the measured flux with wavelength with no apparent silicate feature around 10 μm. We find that the mid-IR emission from the nucleus can be reproduced by an extended dust disk or torus with a small covering factor of the radiating dust clouds. Conclusions. Our mid-IR observations of NGC 3783 are consistent with a clumpy circumnuclear dust environment. The interpretation in terms of a dusty torus with a low covering factor supports a clumpy version of the unified scheme for AGN. The inferred sizes and luminosities are in good agreement with dust reverberation sizes and bolometric luminosities from optical and X-ray observations.

Starbursts and torus evolution in AGN

Recent VLT SINFONI observations of the close environments (∼30 pc) of nearby AGNs have shown that thick gas tori and starbursts with ages between 10 and 150 Myr are frequently found. By applying these observations to a previously established analytical model of clumpy accretion disks, we suggest an evolutionary sequence for starburst and AGN phases. Whereas the observed properties of the gas tell us about the current state of the torus, the starburst characteristics provide information on the history of the torus. In the suggested evolution, a torus passes through 3 different phases predetermined by an external mass accretion rate. Started by an initial, short, and massive gas infall, a turbulent and stellar wind-driven Q ∼ 1 disk is formed in which the starburst proceeds. Once the supernovae explode the intercloud medium is removed, leaving a massive, geometrically thick, collisional disk with a decreasing, but still high-mass accretion rate. When the mass accretion rate has significantly decreased, the collisional torus becomes thin and transparent as the circumnuclear disk in the Galactic center of the Milky Way. Variations on this scenario are possible either when there is a second short and massive gas infall, in which case the torus may switch back into the starburst mode, or when there is no initial short massive gas infall. All observed tori up to now have been collisional and thick. The observations show that this phase can last more than 100 Myr. During this phase the decrease in the mass accretion rate within the torus is slow (a factor of 4 within 150 Myr). The collisional tori also form stars, but with an efficiency of about 10% when compared to a turbulent disk.

High-spatial resolution SED of NGC 1068 from near-IR to radio : Disentangling the thermal and non-thermal contributions

We investigate the idea that a sizable fraction of the interferometrically unresolved infrared emission of the nucleus of NGC 1068 might originate from processes other than thermal dust emission from the torus. We examine the contribution of free-free or synchrotron emission to the central mid- and near-IR parsec-scale emitting region of NGC 1068. Each mechanism is constrained using parsec scale radio data available for NGC 1068 in the 10 9 −10 11 Hz regime, and compared with the highest-resolution interferometric data available in the mid-infrared. It is shown that the unresolved emission in the interferometric observation (<1 pc) is still dominated by dust emission and not by contributions from synchrotron or free-free emission. As previous studies suggest, the interferometric observations appear to infer a clumpy structure of the dust distribution. Extrapolation of the radio free-free or synchrotron emission to the IR indicates that their contribution is <20% even for the unresolved fraction of the interferometric flux. The slope of the available radio data is consistent with a power law exponent α = 0.29 ± 0.07 that we interpret in terms of either free-free emission or synchrotron radiation from quasi-monochromatic electrons. We apply emission models for both mechanisms to obtain physical parameters. Furthermore, we attempt to quantify the possible contribution of the accretion disk to the near-infrared emission. It has been suggested, that the unresolved K-band flux in VLTI/VINCI interferometric observation at 46 m baseline (40% of the total K-band flux) might originate in the accretion disk. By using an accretion disk spectrum that has been adjusted to the luminosity and black hole mass of NGC 1068, we find that the expected accretion-disk flux in the K-band is negligible. Moreover, the scenario of detecting the accretion disk through holes in a clumpy torus is extremely unlikely. We conclude that all current IR data of NGC 1068 trace the torus dust emission, favoring a clumpy torus.

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

Turbulent viscosity in clumpy accretion disks: Application to the Galaxy

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Discovery of a strong Baldwin effect in mid-infrared AGN lines

=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

Where have all the black holes gone

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