arXiv subjects

A. Witzel

Publications and source records attributed to A. Witzel.

At least 55 records · Page 3Linked to original sources

arXiv2005

The Innermost AGNs with Future mm-VLBI

The capabilities of the Global mm-VLBI Array are summarized and demonstrated through actual images from our monitoring of extragalactic radio jets. This sensitive 3mm-VLBI interferometer is able to provide images of up to 50 microarcseconds resolution. For the near future, ALMA, the GBT, the LMT, CARMA, SRT, Yebes, Nobeyama and Noto are some of the most sensitive stations suitable to participate in mm-VLBI. This future array, together with the present Global mm-VLBI Array, would achieve 10 times better sensitivities than nowadays. Image fidelity would also largely increased. T he addition of ALMA would improve the (u,v)-coverage for sources with low declination (<20 deg.) and facilitate the VLBI imaging of the Galactic Centre source SgrA*.

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arXiv2005

Dual frequency VSOP imaging of the jet in S5 0836+710

The luminous high-redshift (z=2.17) quasar S50836+710 has been observed in October 1997 with the VSOP at 1.6 GHz and 5 GHz. We report here a previously unpublished image made from the data at 1.6 GHz and compare the structure of a relativistic jet in this quasazr at the two frequencies. We present a spectral index image tracing spectral properties of the jet up to ~40 milliarcsecond distance from the nucleus. The curved jet ridge line observed in the images and the spectral index distribution can be described by Kelvin-Helmholtz instability developing in a relativistic outflow with a Mach number of ~6. In this description, the overall ridge line of the jet is formed by the helical surface mode of Kelvin-Helmholtz instability, while areas of flatter spectral index embedded into the flow correspond to pressure enhancements produced by the elliptical surface mode of the instability. An alternative explanation involving a sequence of slowly dissipating shocks cannot be ruled out at this point.

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arXiv2005

Kinematic Study of the Blazar S5 0716+714

We present the results of a multi-frequency study of the structural evolution of the VLBI jet in the BL Lac object 0716+714 over the last 10 years. We show VLBI images obtained at 5 GHz, 8.4 GHz, 15 GHz and 22 GHz. The milliarcsecond source structure is best described by a one-sided core-dominated jet of ~10 mas length. Embedded jet components move superluminally with speeds ranging from 5 c to 16 c (assuming z=0.3). Such fast superluminal motion is not typical for BL Lac objects, however it is still in the range of jet speeds typically observed in quasars (10 c to 20 c). In 0716+714, younger components, that were ejected more recently, seem to move systematically slower than the older components. This and a systematic position angle variation of the inner (1 mas) portion of the VLBI jet, suggests an at least partly geometric origin of the observed velocity variations. The observed rapid motion and the derived Lorentz factors are discussed with regard to the rapid Intra-Day Variability (IDV) and the gamma-ray observations, from which very high Doppler factors are inferred.

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arXiv2005

86 GHz polarimetry of OVV1633+382 after a major mm flare

The 18 mag QSO 1633+382 (4C38.41, z=1.807) showed a very pronounced outburst in 2001/2002. With a peak amplitude of more than 9 Jy at 90GHz, this flare was brighter than any known previous flare in this source (data available since 1980).During onset, the mm-flare was particulary fast, with an increase of more than 2 Jy at 230 GHz in less than 8 days. Since January 2002, the mm-flux of 1633+382 is decaying. During this decline, however, local flux variations with amplitudes of 1-3 Jy were seen, indicative of underlying and more rapid source activity on time scales of 1-2 months. After the main peak occurring in 2001.99, the 90 GHz flux showed secondary maxima at approximately half year intervals in 2002.3, 2002.7 and 2003.13. This kind of periodicity might be explained via the lighthouse model (Camenzind and Krockenberger 1992), which is based on the magnetic accelerator of Blanford and Payne (1982). At present the millimeter flux is nearly back to its quiescent level of 2-2.5 Jy, which the source had before the flare began. Our VLBA Polarimetry monitoring started June 2002 during the onset of the flare. At cm wavelength, the flare is only marginally detected which implies very high opacity of the source.

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arXiv2004

Towards the Event Horizon - The Vicinity of AGN at Micro-Arcsecond Resolution

We summarize the present status of VLBI experiments at 3 mm (86 GHz), 2 mm (129-150 GHz) and 1.3 mm (215-230 GHz). We present and discuss a new 3 mm VLBI map of M87 (Virgo A), which has a spatial resolution of only approx. 20 Schwarzschild radii. We discuss recent VLBI results for SgrA* and argue in favor of new observations within an extended European mm-VLBI network, in order to search for variability. We discuss the possibilities to image the `event horizon' of a super-massive black hole at wavelengths < 2mm, and conclude that the addition of large and sensitive millimetre telescopes such as CARMA, the SMA, the LMT and ALMA will be crucial for this.

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arXiv2004

Millimetre-VLBI Monitoring of AGN with Sub-milliarcsecond Resolution

Global millimetre VLBI allows detailed studies of the most central jet regions of AGN with unprecedent spatial resolution of a few 100-1000 Schwartzschild radii to be made. Study of these regions will help to answer the question how the highly relativistic AGN jets are launched and collimated. Since the early 1990s, bright mm-sources have been observed with global 3 mm VLBI. Here we present new images from an ongoing systematic analysis of the available observations. In particular, we focus on the structure and structural evolution of the best observed AGN jets, taking 3C 454.3 as a characteristic example. This core-dominated and highly variable quasar shows a complex morphology with individual jet components accelerating superluminally towards the outer structure. We briefly discuss the X-ray properties of 3C 454.3 and present its radio- to X-ray large-scale brightness distribution.

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arXiv2004

Spectral Properties of the Core and the VLBI-Jets of Cygnus A

We present a detailed VLBI study of the spectral properties of the inner core region of the radio galaxy Cygnus A at 5 GHz, 15 GHz, 22 GHz, 43 GHz and 86 GHz. Our observations include an epoch using phase-referencing at 15 GHz and 22 GHz and the first successful VLBI observations of Cygnus A at 86 GHz. We find a pronounced two-sided jet structure, with a steep spectrum along the jet and an inverted spectrum towards the counter-jet. The inverted spectrum and the frequency-dependent jet-to-counter-jet ratio suggest that the inner counter-jet is covered by a circum-nuclear absorber as it is proposed by the unified scheme.

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arXiv2004

Motion and Properties of Nuclear Radio Components in Seyfert Galaxies Seen with VLBI

We report EVN, MERLIN and VLBA observations at 18 cm, 6 cm and 3.6 cm of the Seyfert galaxies NGC 7674, NGC 5506, NGC 2110 and Mrk 1210 to study their structure and proper motions on pc scales and to add some constraints on the many possible causes of the radio-quietness of Seyferts. The component configurations in NGC 7674 and NGC 2110 are simple, linear structures, whereas the configurations in NGC 5506 and Mrk 1210 have multiple components with no clear axis of symmetry. We suggest that NGC 7674 is a low-luminosity compact symmetric object. Comparing the images at different epochs, we find a proper motion in NGC 7674 of (0.92+-0.07) c between the two central components separated by 282 pc and, in NGC 5506, we find a 3 sigma upper limit of 0.50 c for the components separated by 3.8 pc. Our results confirm and extend earlier work showing that the outward motion of radio components in Seyfert galaxies is non-relativistic on pc scales. We briefly discuss whether this non-relativistic motion is intrinsic to the jet-formation process or results from deceleration of an initially relativistic jet by interaction with the pc or sub-pc scale interstellar medium. We combined our sample with a list compiled from the literature of VLBI observations made of Seyfert galaxies, and found that most Seyfert nuclei have at least one flat-spectrum component on the VLBI scale, which was not seen in the spectral indices measured at arcsec resolution. We found also that the bimodal alignment of pc and kpc radio structures displayed by radio galaxies and quasars is not displayed by this sample of Seyferts, which shows a uniform distribution of misalignment between 0 deg and 90 deg. The frequent misalignment could result from jet precession or from deflection of the jet by interaction with gas in the interstellar medium.

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arXiv2003

Quasi-Periodic Changes in the Parsec-Scale Jet of 3C345

We have observed the QSO 3C345 with VSOP at 1.6GHz and 5GHz and with the VLBA at 22GHz and 43GHz, with 24 epochs in total, from 1997.40 to 1999.69. This study, combined with previous observations, reveals periodic changes in the parsec-scale jet. This quasi-periodicity can be found in the initial position angles, the trajectories and the flux density evolution of the jet components, with a period of about 9 years. This period should reflect the dynamics of the material in the central region of 3C345 and it is possibly related to a putative binary black hole in the center of this object.

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arXiv2003

Approaching NGC3079 with VLBI

The Seyfert 2 galaxy NGC3079 is one of the nearest radio-weak AGN. We have carried out multi-frequency, multi-epoch VLBI observations of this object and detected new components, proper motion of the order of 0.1 c, strong spectral variability, and free-free absorption. Based on phase-referenced astrometry, we suggest a new identification of components seen at 1.7 GHz. Our results suggest that the nucleus in NGC3079 is embedded in a dense medium which strongly affects the radio appearance of the AGN ejecta. We supplement our work with Seyfert VLBI observations taken from the literature.

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arXiv2003

Multi-frequency & Multi-epoch VLBI study of Cygnus A

We present the first multi-frequency phase-referenced observations of Cygnus A done with the VLBA at 15 and 22 GHz. We find a pronounced two-sided jet structure, with a steep spectrum along the jet and a highly inverted spectrum towards the counter-jet. The inverted spectrum and the frequency dependent jet to counter-jet ratio suggest an obscuring torus in front of the counter-jet. From 14 epochs of 15 GHz VLBA data we accurately derive the jet and counter-jet kinematics. For the inner jet (r <= 5 mas) we measure motions of beta_app ~ 0.2-0.5 h^{-1} and on the counter-jet side we find beta_ app ~ 0.03+-0.02 h^{-1}. We discuss the jet velocities within the unified jet model.

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arXiv2003

Kinematic Study of the Blazar 0716+714

We determined the kinematics at the jet of 0716+714 from a reanalysis of multi-frequency VLBI data (5, 8.4, 15, 22 GHz) obtained during the last 10 years combined with data from the literature. For this intra-day variable blazar, only a lower limit of its distance is known (z >= 0.3). We find that 0716+714 is a relatively fast superluminal source (with a Lorentz factor of gamma >15), revising earlier results showing much slower motions. We discuss the new findings with emphasis on the interpretation of the observed rapid radio variability.

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arXiv2003

Millimeter VLBI and Variability in AGN Jets

Millimeter-VLBI images probe as deep as never before the nuclei of AGN. Global 3mm-VLBI where the VLBA is combined with sensitive European stations, gives CLEAN maps of a quality, which never before was so good. VLBI at 147 GHz yields transatlantic fringes for the first time. Now it will become possible to study the relation between jet kinematics and spectral activity with a few ten micro-arcsecond resolution at the shortest wavelengths.

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arXiv2003

Optical and radio behaviour of the BL Lacertae object 0716+714

Eight optical and four radio observatories have been intensively monitoring the BL Lac object 0716+714 in the last years: 4854 data points have been collected in the UBVRI bands since 1994, while radio light curves extend back to 1978. Many of these data are presented here for the first time. The long-term trend shown by the optical light curves seems to vary with a characteristic time scale of about 3.3 years, while a longer period of 5.5-6 years seems to characterize the radio long-term variations. In general, optical colour indices are only weakly correlated with brightness. The radio flux behaviour at different frequencies is similar, but the flux variation amplitude decreases with increasing wavelength. The radio spectral index varies with brightness (harder when brighter), but the radio fluxes seem to be the sum of two different-spectrum contributions: a steady base level and a harder-spectrum variable component. Once the base level is removed, the radio variations appear as essentially achromatic, similarly to the optical behaviour. Flux variations at the higher radio frequencies lead the lower-frequency ones with week-month time scales. The behaviour of the optical and radio light curves is quite different, the broad radio outbursts not corresponding in time to the faster optical ones and the cross-correlation analysis indicating only weak correlation with long time lags. However, minor radio flux enhancements simultaneous with the major optical flares can be recognized, which may imply that the mechanism producing the strong flux increases in the optical band also marginally affects the radio one.

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arXiv2002

Compact Intraday Variable Radio Cores: New Observational Approaches

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 become stronger. If IDV is still a complex composition of extrinsic and source intrinsic effects, the intrinsic part of the IDV pattern should show up in the millimeter and sub-millimeter regime due to the frequency dependence of RISS. Hence, observations at higher frequencies are essential in order to exclude RISS as the sole cause of IDV. Here we report on our new attempt to search for rapid variations at much higher frequencies. In addition, the possibility of a direct detection of the postulated scattering screen in front of IDV sources will be discussed. Our recent line observations towards a few IDV sources lead to the first detection of a high latitude molecular cloud in front of an intraday variable radio core.

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arXiv2002

2mm Wavelength VLBI of SiO Masers and AGN

In April 2002 an array of antennas operating at 129GHz successfully detected VLBI fringes on both continuum AGN and SiO maser sources. The 129GHz fringes on maser sources represent the highest frequency spectral line VLBI detections to date. The AGN 3C279 was detected on long baselines at both 129GHz (and at 147GHz, see Krichbaum et al in these proceedings) yielding fringe spacings of 50-56 micro arc seconds, an angular resolution record. The array consisted of the University of Arizona Kittpeak 12m antenna, the Heinrich Hertz 10m Telescope (HHT), and the IRAM 30m dish on Pico Veleta. At 129GHz, a number of evolved stars and several young stellar objects exhibit strong SiO maser emission in the v=1 J=3-2 transition. Preliminary cross power spectra of SiO masers around the red hypergiant VYCMa on the HHT-KittPeak baseline ~190km are consistent with multiple spatially separate maser spots associated with the star. Future observations will include continuum observations of the radio source at the Galactic Center, SgrA*, and higher frequency maser lines including HCN and methanol.

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arXiv2002

Proper motion in Cygnus A

Our recent VLBI observations of the prominent FR II radio galaxy Cygnus A with the EVN and the VLBA reveal a pronounced two-sided jet structure. At 5 GHz, we now have 4 epochs from 1986, 1991 (Carilli et al., 1991 & 1994), 1996 and 2002 from which we could derive the kinematics of the jet and counter-jet. On the jet side and on mas scales, the jet seems to accelerate from $β_{\rm app}\approx 0.1-0.2$ (Krichbaum et al. 1998) at core-separations near 1 mas to $β_{\rm app}\approx 0.4-0.6$ at $r \geq 4$ mas ($H_0=100$ km s$^{-1}$ Mpc$^{-1}$, $q_0=0.5$). For the first time we also measure significant structural variability on the counter-jet side. For this, we derive a motion of $β_{\rm app}=0.35\pm0.2$ at $r=9.5$ mas. The flat spectrum of the inner region of the counter-jet (free-free absorption) and the frequency dependence of the jet to counter-jet ratio support strong evidence for an obscuring torus in front of the counter-jet (Bach et al. in prep, Krichbaum et al. 1998).

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arXiv2002

Is 0716+714 a superluminal blazar?

We present an analysis of new and old high frequency VLBI data collected during the last 10 years at 5--22 GHz. For the jet components in the mas-VLBI jet, two component identifications are possible. One of them with quasi-stationary components oscillating about their mean positions. Another identification scheme, which formally gives the better expansion fit, yields motion with $\sim 9$ $c$ for $H_0=65$ km s$^{-1}$ Mpc$^{-1}$ and $q_0=0.5$. This model would be in better agreement with the observed rapid IDV and the expected high Lorentz-factor, deduced from IDV.

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