Publications (24)
VLTI/VINCI observations of the nucleus of NGC 1068 using the adaptive optics system MACAO
M. Wittkowski, P. Kervella, R. Arsenault +3
We present the first near-infrared K-band long-baseline interferometric measurement of the prototype Seyfert 2 galaxy NGC 1068 with resolution lambda/B \~ 10 mas obtained with the…
Annual Modulation in the Variability Properties of the IDV-Source 0917+624 ?
L. Fuhrmann, T. P. Krichbaum, G. Cimò +6
There is new evidence which identifies seasonal changes of the variability time scale in intraday variable sources with refractive interstellar scintillation effects. Such a RISS-m…
Where has all the polarization gone?
E. Middelberg, A. L. Roy, U. Bach +2
We have observed the five radio galaxies NGC1052, NGC4261, Hydra A, Centaurus A and Cygnus A with polarimetric VLBI at 15 GHz to look for possible Faraday rotation in foreground ab…
Multi-frequency Study of Intraday Variable Sources
Giuseppe Cimò, L. Fuhrmann, T. P. Krichbaum +4
Intraday variability (IDV) of compact extragalactic radio sources is a complex phenomenon and shows a wavelength dependent mixture of refractive interstellar scintillation (RISS) (…
Milli-arcsecond astrophysics with VSI, the VLTI spectro-imager in the ELT era
F. Malbet, D. Buscher, G. Weigelt +49
Nowadays, compact sources like surfaces of nearby stars, circumstellar environments of stars from early stages to the most evolved ones and surroundings of active galactic nuclei c…
From the Circumnuclear Disk in the Galactic Center to thick, obscuring tori of AGNs
B. Vollmer, T. Beckert, W. J. Duschl
We compare three different models of clumpy gas disk and show that the Circumnuclear Disk (CND) in the Galactic Center and a putative, geometrically thick, obscuring torus are best…
A very rapid Extreme Scattering Event in the IDV source 0954+658
G. Cimò, T. Beckert, T. P. Krichbaum +4
Extreme Scattering Events (ESEs) are dramatic variations of the flux density at Gigahertz frequencies caused by ray path distortions within an isolated inhomogeneity ("plasma lens"…
Diffraction-limited 76 mas Speckle Masking Observations of the Core of NGC 1068 with the SAO 6m Telescope
M. Wittkowski, Y. Balega, T. Beckert +3
We present the first K-band bispectrum speckle interferometry of NGC 1068 with an angular resolution of 76 mas (~5.5pc). This angular resolution allows us to attribute the measured…
Compact Intraday Variable Radio Cores: New Observational Approaches
L. Fuhrmann, G. Cimò, T. P. Krichbaum +4
The evidence for refractive interstellar scintillation (RISS) being the main cause for rapid intraday variations (Intraday Variability, IDV) in Quasars and BL Lacs has recently bec…
Where have all the black holes gone?
T. Beckert, W. J. Duschl
We have calculated stationary models for accretion disks around super-massive black holes in galactic nuclei. Our models show that below a critical mass flow rate of ~3 10**-3 M_Ed…
Understanding Scintillation of Intraday Variables
T. Beckert, L. Fuhrmann, G. Cimo +3
Intraday Variability of compact extragalactic radio sources can be interpreted as quenched scintillation due to turbulent density fluctuations of the nearby ionized interstellar me…
Discovery of a strong Baldwin effect in mid-infrared AGN lines
S. F. Hoenig, A. Smette, T. Beckert +5
We present the discovery of a Baldwin effect in 8 nearby Seyfert galaxies for the three most prominent mid-infrared forbidden emission lines observable from the ground that are com…
3D Radiative Transfer Modeling of Clumpy Dust Tori Around AGN
S. F. Hoenig, T. Beckert, K. Ohnaka +1
We present 3-dimensional radiative transfer models for clumpy dust tori around AGN. Our method combines Monte Carlo simulations of individual dust clouds with the actual 3-dimensio…
Radiative Transfer Modeling of Three-Dimensional Clumpy AGN Tori and its Application to NGC 1068
S. F. Hoenig, T. Beckert, K. Ohnaka +1
Recent observations of NGC 1068 and other AGN support the idea of a geometrically and optically thick dust torus surrounding the central supermassive black hole and accretion disk…
Circular Polarimetry of Extragalactic Radio Sources
E. Cenacchi, A. Kraus, T. Beckert +1
We report multi-frequency circular polarization measurements for the four extragalactic radio sources 0056-00, 0716+71, 3C138 and 3C161 taken at the Effelsberg 100-m radiotelescope…
Turbulent viscosity in clumpy accretion disks II supernova driven turbulence in the Galaxy
B. Vollmer, T. Beckert
An analytical model for a turbulent clumpy gas disk is presented where turbulence is maintained by the energy input due to supernovae. Expressions for the disk parameters, global f…
VSI: the VLTI spectro-imager
F. Malbet, D. Buscher, G. Weigelt +51
The VLTI Spectro Imager (VSI) was proposed as a second-generation instrument of the Very Large Telescope Interferometer providing the ESO community with spectrally-resolved, near-i…
Synchrotron radiation from quasi-monoenergetic electrons - Modelling the spectrum of Sgr A*
T. Beckert, W. J. Duschl
We investigate the spectrum of a quasi-monoenergetic ensemble of relativistic electrons, especially for the mildly relativistic case, and discuss the effect of inclination of the m…
The kinematics in the pc-scale jets of AGN The case of S5 1803+784
S. Britzen, N. A. Kudryavtseva, A. Witzel +8
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.…
Starbursts and torus evolution in AGN
B. Vollmer, T. Beckert, R. Davies
Recent VLT SINFONI observations of the close environments (~30pc) of nearby AGNs have shown that thick gas tori and starbursts with ages between 10 and 150Myr are frequently found.…
Turbulent viscosity in clumpy accretion disks. Application to the Galaxy
B. Vollmer, T. Beckert
The equilibrium state of a turbulent clumpy gas disk is analytically investigated. The disk consists of distinct self-gravitating clouds. Gravitational cloud-cloud interactions tra…
Advection-dominated Accretion in the Galactic Center and the Appearance of Sgr A*
T. Beckert, W. J. Duschl
We present a modified description of advection-dominated accretion flows (ADAF) with a geometrical viscosity. A simplified treatment of the dynamics of the flow in the inner relati…
The Size of IDV Jet Cores
T. Beckert, T. P. Krichbaum, G. Cimo' +4
Radio variability on timescales from a few hours to several days in extragalactic flat-spectrum radio sources is generally classified as intra-day variability (IDV). The origin of…
Models of Polarized and Variable Radio Emission for IDV Source 0917+624
T. Beckert, A. Kraus, T. P. Krichbaum +2
We examine the power spectra of IDV and show the information, which is to be gained by wavelet analysis of light curves of the quasar 0917+624. Results for total and polarized flux…