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A. Witzel

Publications and source records attributed to A. Witzel.

At least 73 records · Page 4Linked to original sources
arXiv2002

Understanding Scintillation of Intraday Variables

Intraday Variability of compact extragalactic radio sources can be interpreted as quenched scintillation due to turbulent density fluctuations of the nearby ionized interstellar medium. We demonstrate that the statistical analysis of IDV time series contains both information about sub-structure of the source on the scale of several 10 micro-arcsec and about the turbulent state of the ISM. The source structure and ISM properties cannot be disentangled using IDV observations alone. A comparison with the known morphology of the `local bubble' and the turbulent ISM from pulsar observations constrains possible source models. We further argue that earth orbit synthesis fails for non-stationary relativistic sources and no reliable 2D-Fourier reconstruction is possible.

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arXiv2002

VLBI observations at 147 GHz: first detection of transatlantic fringes in bright AGN

At 147 GHz (2mm wavelength), we detected three prominent AGN (NRAO150, 3C279, 1633+382) with Very Long Baseline Interferometry (VLBI) with an angular resolution of only 18 micro-arcseconds on the baseline between two antennas in Arizona (10m HHT and 12m KittPeak) and the IRAM 30m antenna on Pico Veleta in Spain. This is a new world record in radio interferometry and astronomical imaging and opens fascinating future possibilities to directly image and study the innermost regions in Quasars and other Active Galactic Nuclei.

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arXiv2002

Multi-frequency Study of Intraday Variable Sources

Intraday variability (IDV) of compact extragalactic radio sources is a complex phenomenon and shows a wavelength dependent mixture of refractive interstellar scintillation (RISS) (dominant at long cm-wavelengths) and source-intrinsic effects (dominant at shorter wavelengths). Detailed investigations of individual sources and new high frequency observations suggest a source-intrinsic contribution to the IDV pattern at least in some sources. However, the sizes of intraday variable sources at cm-wavelength are typically smaller than the scattering size set by the ISM in our galaxy and scintillation must be present, too. We present new IDV observations in different regimes (from cm to sub-mm wavelengths) and show how such multi-frequency study can be used as powerful instrument to describe different aspects of IDV.

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arXiv2002

The Role of Millimeter VLBI Observations in AGN Research

VLBI at millimeter wavelengths (mm-VLBI) allows the detailed imaging of compact galactic and extragalactic radio sources with micro-arcsecond scale resolution, unaccessible by other observing techniques. Here we discuss the scientific potential of mm-VLBI for present and future research on `Active Galactic Nuclei' (AGN) and their powerful relativistic jets. With the new generation of large radio telescopes and interferometer arrays operating in the millimeter radio bands (e.g. ALMA), the ultimate vicinity of super massive Black Holes, and eventually even their event horizon, could be imaged. With its large collecting area, and in combination with these future telescopes, the Sardinia Radio Telescope could form the World's `sharpest' astronomical imaging machine.

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arXiv2001

The Size of IDV Jet Cores

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 this short term variability is still controversial and both extrinsic and intrinsic mechanisms must be considered and may both contribute to the observed variations. The measured linear and circular polarization of IDV sources constrains the low energy end of the electron population. Any population of cold electrons within sources at or above the equipartition temperature of 10^11 K depolarizes the emission and can be ruled out. Intrinsic shock models are shown to either violate the large fraction of sources displaying IDV or they do not relax the light travel time argument for intrinsic variations. From structure function analysis, we further conclude that interstellar scintillation also leads to tight size estimates unless a very local cloud in the ISM is responsible for IDV.

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arXiv2001

Annual Modulation in the Variability Properties of the IDV-Source 0917+624 ?

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-model takes the annual change of the Earth velocity relative to the scattering medium into account. In September 1998 we found a remarkable prolongation of the variability time scale in the IDV-source 0917+624 with only small variations in flux density during a period of 5 days. This was explained as a seasonal effect, in which the velocity vector of the Earth and the interstellar medium nearly canceled. In order to further investigate the applicability of the model for 0917+624, we performed an Effelsberg 6cm-flux monitoring program over the course of one year. Since September 2000, the source appears to be remarkably inactive and yet (May 2001), no return to its normal, faster and stronger variability pattern is observed. Here, our observational results and a possible explanation for the current quiescence are presented.

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arXiv2001

Intraday Variability in Northern Hemisphere Radio Sources

We summarize results from flux density monitoring campaigns performed with the 100 meter radio-telescope at Effelsberg and the VLA during the past 15 yrs. We briefly discuss some of the statistical properties of the rapid variability from now more than 40 high declination sources, which show Intraday Variability (IDV). In general, IDV is more pronounced for sources with flat radio spectra and compact VLBI structures. The frequency dependence of the variability pattern varies with source and observing time. For 0917+62, we present new VLBI images, which suggest that the variability pattern is modified by the occurrence of new jet components. For 0716+71, we show the first detection of IDV at millimeter wavelengths (32 GHz). For the physical interpretation of the IDV phenomenon, a complex source and frequency dependent superposition of interstellar scintillation and source intrinsic variability should be considered.

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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.

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arXiv2001

A High-Frequency and Multi-Epoch VLBI Study of 3C 273

We show results from a 7 year VLBI monitoring programme of 3C273 at millimeter wavelengths. We find evidence for component acceleration, motion or rotation of fluid dynamical patterns in the VLBI jet, and evidence for an outburst-ejection relation between Gamma-ray flares and the appearance of new jet components.

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arXiv2001

Annual modulation in IDV of quasar 0917+624 due to Interstellar Scintillation

The quasar 0917+624 has been one of the best studied intraday variable (IDV) radio sources. However, debate continues as to whether the underlying cause is intrinsic or extrinsic. Much previous work has assumed the IDV to be intrinsic and which implies an extraordinarily compact source for the radio emission; in contrast, an extrinsic variation due to interstellar scintillation (ISS) implies a relatively larger source diameter, though at the smaller end of the range expected for relativistic jet models. Kraus et al. (1999) reported a marked slowing of the IDV at 6cm wavelength in September 1998, and suggested a change in the source was responsible. However, here we show that the slowing is consistent with the annual modulation in scintillation time-scale expected for ISS, under the assumption that the scattering medium moves with the local standard of rest (LSR). The ISS time scale is governed by the ISS spatial scale divided by the Earth's velocity relative to the scattering plasma. It happens that in the direction of 0917+624 the transverse velocity of the Earth with respect to the LSR varies widely with a deep minimum in the months of September to November. Hence the slowing of the IDV in September 1998 strongly suggests that ISS, rather than intrinsic variation of the source is the dominant cause of the IDV.

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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.

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arXiv1999

High Angular Resolution Monitoring of Prominent AGN at 86 GHz

We report results from 3mm-VLBI observations of 3C273 and 3C454.3 during the period 1994-1997. In both sources the images show superluminal motion in feature-rich jets. Due to the improved uv-coverage in the later experiments, the jets can be followed further out than previously. Comparision with maps obtained at longer wavelengths allows to determine spectral gradients along the jets. Opacity effects lead to transverse displacements of the jet ridgelines. These displacements appear to be different in 3C273 and 3C454.3.

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arXiv1999

SKA in VLBI: Impact on Studies of Small Scale Structures in Active Galactic Nuclei

We discuss the advantages which the next generation interferometer SKA (the Square Kilometer Array) will bring for Very Long Baseline Interferometry (VLBI) at centimeter wavelengths. With a sensitivity to micro-Jy level flux density, a brightness temperature limit of about 10.000 K, and an expected dynamic range in future VLBI maps of better than 1.000.000, the impact of SKA on the research of compact extragalactic radio sources and their energetic jets will be very strong. We discuss some consequences, including a possible combination of SKA with future space VLBI missions.

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arXiv1999

High precision difference astrometry applied to the triplet of S5 radio sources B1803+784/Q1928+738/B2007+777

We determined the separations of the radio sources in the triangle formed by the BL Lac objects 1803+784 and 2007+777, and the QSO 1928+738 from intercontinental interferometric observations carried out in November 1991 at the frequencies of 2.3 and 8.4 GHz simultaneously. We successfully connected differenced phase delays over 7^o on the sky at 8.4 GHz at an epoch of maximum solar activity. The effects of the ionosphere on these VLBI data were mostly removed by estimates of the total electron content from observations of GPS satellites. The comparison of the estimated separation of QSO 1928+738 and BL 2007+777 with previous such estimates obtained from data at different epochs leads us to a particular alignment of the maps of QSO 1928+738 at different epochs relative to those of BL 2007+777, although with significant uncertainty. For this alignment, the jet components of QSO 1928+738 show a mean proper motion of 0.32+/-0.10 mas/yr and also suggest an identification for the position of the core of this radio source.

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arXiv1999

Intraday Radio Variability in Active Galactic Nuclei

Rapid flux density variations on timescales of the order of a day or less (Intraday Variability, IDV) in the radio regime are a common phenomenon within the blazar class. Observations with the 100m telescope of the MPIfR showed that the variations occur not only in total intensity, but also in the polarized intensity and in polarization angle. Here we present an overview of our IDV-observations and discuss briefly some models which may explain the effect.

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arXiv1999

Unusual radio variability in the BL Lac object 0235+164

We present radio observations at three frequencies and contemporaneous optical monitoring of the peculiar BL Lac object AO 0235+164. During a three-week campaign with the VLA we observed intraday variability in this source and found a distinct peak which can be identified throughout the radio frequencies and tentatively connected to the R-band variations. This event is characterized by unusual properties: its strength increases, and its duration decreases with wavelength, and it peaks earlier at 20cm than at 3.6 and 6cm. We discuss several generic models (a `standard' shock-in-jet model, a precessing beam, free-free-absorption in a foreground screen, interstellar scattering, and gravitational microlensing), and explore whether they can account for our observations. Most attempts at explaining the data on 0235+164 require an extremely small source size, which can be reconciled with the $10^{12}$ K inverse Compton limit only when the Doppler factor of the bulk flow is of order 100. However, none of the models is completely satisfactory, and we suggest that the observed variability is due to a superposition of intrinsic and propagation effects.

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arXiv1998

VLBI Observations of the Galactic Center Source Sgr A* at 86 GHz and 215 GHz

We summarize previous VLBI observations of Sgr A* at millimeter wavelengths and present new results from VLBI observations at 86 and at 215 GHz. At 86 GHz the measured closure phase is close to zero, consistent with a point-like or symmetric structure of 190 +/- 30 micro-arcsec size. At 215 GHz we have detected Sgr A* with a signal-to-noise ratio of 6. This yields a tentative size estimate of 50 < theta < 190 micro-arcsec, which is larger than the scattering size of 20 micro-arcsec at this frequency. The intrinsic size of Sgr A* thus appears to be a few up to a few ten Schwarzschild radii.

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