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T. Beckert

Publications and source records attributed to T. Beckert.

24 records · Page 2Linked to original sources
arXiv2001

Turbulent viscosity in clumpy accretion disks. Application to the Galaxy

The equilibrium state of a turbulent clumpy gas disk is analytically investigated. The disk consists of distinct self-gravitating clouds. Gravitational cloud-cloud interactions transfer energy over spatial scales and produce a viscosity, which allows mass accretion in the gas disk. Turbulence is assumed to be generated by instabilities involving self-gravitation and to be maintained by the energy input from differential rotation and mass transfer. Disk parameters, global filling factors, molecular fractions, and star formation rates are derived. The application of our model to the Galaxy shows good agreement with observations. They are consistent with the scenario where turbulence generated and maintained by gravitation can account for the viscosity in the gas disk of spiral galaxies. The role of the galaxy mass for the morphological classification of spiral galaxies is investigated.

astro-ph↗
arXiv2001

A very rapid Extreme Scattering Event in the IDV source 0954+658

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") in the interstellar medium. These events are characterized by a deep flux density minimum in the light curve with, in some cases, surrounding maxima. The variability time scales range from weeks to months. These phenomena show a strong frequency dependence, in which the variability amplitudes increase with wavelength. During an Intraday Variability (IDV) monitoring project (March 2000), a feature resembling an ESE-like event appeared in the variable light curve of 0954+658, however with a time scale of less than two days. We will discuss this effect and its implications for a better description of the interstellar medium.

astro-ph↗
arXiv2000

Models of Polarized and Variable Radio Emission for IDV Source 0917+624

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 at 11cm are shown. Both interstellar scattering and intrinsic models have difficulties in explaining the 1 day period variations. A theoretical model for the time averaged emission is presented, which provides the basis for the analysis of possible variations.

astro-ph↗
arXiv1997

Diffraction-limited 76 mas Speckle Masking Observations of the Core of NGC 1068 with the SAO 6m Telescope

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 flux to only one of the nuclear sources seen at radio wavelengths. The observed decreasing visibility function suggests that the dominant central core is probably not an unresolved point source, but slightly resolved with a FWHM diameter of ~30 mas ~2pc for an assumed Gaussian intensity distribution. This 30 mas object is possibly the nuclear torus and/or a scattering halo. We discuss different contributions to the observed K band flux. Between 5 GHz and the K-band the spectrum of this component is close to a nu^(1/3) proportionality. In addition to the standard interpretation of a hot dust torus surrounding the nucleus of NGC 1068, one cannot exclude the possibility that a sizeable fraction of the nuclear flux reaches us via a scattering halo. This then would allow us to determine physical parameters of the nuclear source.

astro-ph↗
arXiv1997

Synchrotron radiation from quasi-monoenergetic electrons - Modelling the spectrum of Sgr A*

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 magnetic field on the emissivity. We apply the exact theoretical description to the spectrum of the radio source Sgr A* which is located at or very close to the dynamical center of our Galaxy. We find that the radio-MIR spectrum can be reproduced well, but that the resulting self-comptonized X-ray flux is much smaller than the observed one.

astro-ph↗