arXiv subjects

A. Kraus

Publications and source records attributed to A. Kraus.

85 records · Page 5Linked to original sources

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.

astro-ph↗

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.

astro-ph↗

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.

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↗

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.

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↗

arXiv2000

Multifrequency Observations of the Virgo Blazars 3C 273 and 3C 279 in CGRO Cycle 8

We report first observational results of multifrequency campaigns on the prominent Virgo blazars 3C 273 and 3C 279 which were carried out in January and February 1999. Both blazars are detected from radio to gamma-ray energies. We present the measured X- to gamma-ray spectra of both sources, and for 3C 279 we compare the 1999 broad-band (radio to gamma-ray) spectrum to measured previous ones.

astro-ph↗

arXiv1999

Mechanical Mixing in Nonlinear Nanomechanical Resonators

Nanomechanical resonators, machined out of Silicon-on-Insulator wafers, are operated in the nonlinear regime to investigate higher-order mechanical mixing at radio frequencies, relevant to signal processing and nonlinear dynamics on nanometer scales. Driven by two neighboring frequencies the resonators generate rich power spectra exhibiting a multitude of satellite peaks. This nonlinear response is studied and compared to $n^{th}$-order perturbation theory and nonperturbative numerical calculations.

cond-mat.mes-hall↗

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.

astro-ph↗

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.

astro-ph↗

arXiv1999

Radiospectra and kinematics in blazars

Broad band spectra of total and compact--scale radio emission from blazars, used in combination with kinematic information inferred from VLBI monitoring programs, can be applied successfully to better constrain the models of compact jets. We discuss here a ``hands-on'' approach for tying together the kinematic and spectral properties of radio emission from blazars.

astro-ph↗

arXiv1998

VSOP imaging of S5 0836+710: a close-up on plasma instabilities in the jet

The luminous high-redshift (z=2.17) quasar S5 0836+710 has been observed at 5GHz with the VSOP. We compare the properties of three images obtained from the observation: a low-resolution ground array image (dynamic range 4600:1), a full-resolution VSOP image (900:1), and an image made with only the space baselines (200:1). The space baselines alone are sufficient for a reliable recovery of the source structure, within the limits of the achieved spatial sampling of the visibility data. The curved jet ridge line observed in the images can be described by Kelvin-Helmholtz instabilities developing in a relativistic outflow with the Mach number of about 6. This description holds on the scales of up to 700h^-1 pc, and is shown to be consistent with variable apparent speeds observed in the jet.

astro-ph↗

arXiv1998

Gamma-ray to radio activity and ejection of a VLBI component in the jet of the S5-quasar 0836+710

Broad-band (gamma to radio) variations of the flux density were observed in the first half of 1992 in the luminous high redshift (z = 2.172) quasar S5 0836+710. VLBI monitoring observations during 1993 -- 1996 performed at 86 GHz, 22 GHz, 15 GHz, and 8 GHz show the ejection of a new jet component, which most probably is directly related to a quasi simultaneous gamma-, X-ray, optical flaring activity which was observed in February 1992. During the period 1992 -- 1993 the flaring propagated through the radio spectrum. From several quasi-simultaneous radio spectra taken during this phase of activity, we determine the time evolution of the spectral turnover of the radio spectrum in the S_m-ν_m diagram. The data indicate a correlation of the jet activity with the variability of the broad-band electromagnetic spectrum of the source. The observational findings are discussed in the framework of relativistic shock models.

astro-ph↗