Demonstration of polarisation calibration with the LBA on Selected AGNs

2019 ◽  
Vol 36 ◽  
Author(s):  
Thanapol Chanapote ◽  
Richard Dodson ◽  
Maria Rioja ◽  
Kitiyanee Asanok ◽  
Jamie Stevens ◽  
...  
Keyword(s):  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
High Quality ◽  
Circular Polarisation ◽  
Long Baseline ◽  
Orientation Model ◽  
Heterogeneous Nature ◽  
Linear Polarisation ◽  
Station Record ◽  
Polarisation Measurements

Abstract We present here the results from a full polarisation study, an important VLBI capability, of a selected set of bright Active Galactic Nuclei (AGNs), along with the steps required to fully calibrate the Australian Long Baseline Array. We compare strategies for high-precision polarisation measurements using two polarisation correction methods: (1) Linear model and (2) Ellipticity-Orientation model and two data-recording techniques: recording (1) nominally circular polarisation at all stations and (2) mixed polarisation, where all but one station record circular polarisation and the other recorded linear polarisation. The latter is corrected post-correlation. We explored these possible solution to discover which will best accommodate the heterogeneous nature of the Australian Long Baseline Array without impacting on the science results. The targets, all compact and of low polarisation fraction, allow us to compare multiple independent solutions for polarisation characteristics. The results show that the agreement between the two polarisation correction models is excellent. However, the values from Mopra with nominally circular polarisation are larger than would be acceptable. However, we also demonstrate that recording mixed polarisation modes and correcting post-correlation provide a high quality polarisation product. We report on the detailed tests of these strategies and assess that the array is ready for full polarisation operation.

scholarly journals Polarisation Characteristics and Variability of the Compact Radio Structures of AGN

2002 ◽  
Vol 19 (1) ◽  
pp. 147-151 ◽  
Author(s):  
Denise C. Gabuzda
Keyword(s):  
Magnetic Field ◽  
Active Galactic Nuclei ◽  
Very Long Baseline Interferometry ◽  
Galactic Nuclei ◽  
Long Baseline ◽  
Polarisation Measurements

AbstractVery long baseline interferometry (VLBI) polarisation measurements provide information about the parsec-scale magnetic field structures in compact active galactic nuclei (AGN), as well as the densities of relativistic and thermal electrons in the radio emitting regions. This paper reviews the role of polarisation VLBI in studies of AGN variability on both relatively long and short (intraday) timescales.

scholarly journals Proper motion in lensed radio jets at redshift 3: A possible dual super-massive black hole system in the early Universe

2019 ◽  
Vol 630 ◽  
pp. A108 ◽  
Author(s):  
C. Spingola ◽  
J. P. McKean ◽  
D. Massari ◽  
L. V. E. Koopmans
Keyword(s):  
Active Galactic Nuclei ◽  
Early Universe ◽  
Gravitational Lensing ◽  
Proper Motion ◽  
Galactic Nuclei ◽  
Massive Black Hole ◽  
Radio Jets ◽  
Long Baseline ◽  
Multi Wavelength ◽  
Low Probability

In this paper, we exploit the gravitational lensing effect to detect proper motion in the highly magnified gravitationally lensed source MG B2016+112. We find positional shifts up to 6 mas in the lensed images by comparing two Very Long Baseline Interferometric (VLBI) radio observations at 1.7 GHz that are separated by 14.359 years, and provide an astrometric accuracy of the order of tens of μas. From lens modelling, we exclude a shift in the lensing galaxy as the cause of the positional change of the lensed images, and we assign it to the background source. The source consists of four sub-components separated by ∼175 pc, with proper motion of the order of tens μas yr−1 for the two components at highest magnification (μ ∼ 350) and of the order of a few mas yr−1 for the two components at lower magnification (μ ∼ 2). We propose single active galactic nuclei (AGN) and dual AGN scenarios to explain the source plane. Although, the latter interpretation is supported by the archival multi-wavelength properties of the object. In this case, MG B2016+112 would represent the highest redshift dual radio-loud AGN system discovered thus far, and would support the merger interpretation for such systems. Also, given the low probability (∼10−5) of detecting a dual AGN system that is also gravitationally lensed, if confirmed, this would suggest that such dual AGN systems must be more abundant in the early Universe than currently thought.

scholarly journals Core-shifts and proper-motion constraints in the S5 polar cap sample at the 15 and 43 GHz bands

2018 ◽  
Vol 614 ◽  
pp. A74 ◽  
Author(s):  
F. J. Abellán ◽  
I. Martí-Vidal ◽  
J. M. Marcaide ◽  
J. C. Guirado
Keyword(s):  
Active Galactic Nuclei ◽  
Very Long Baseline Interferometry ◽  
Galactic Nuclei ◽  
Polar Cap ◽  
Motion Constraints ◽  
The Core ◽  
Long Baseline ◽  
Precision Phase ◽  
First Time ◽  
Core Shift

We have studied a complete radio sample of active galactic nuclei with the very-long-baseline-interferometry (VLBI) technique and for the first time successfully obtained high-precision phase-delay astrometry at Q band (43 GHz) from observations acquired in 2010. We have compared our astrometric results with those obtained with the same technique at U band (15 GHz) from data collected in 2000. The differences in source separations among all the source pairs observed in common at the two epochs are compatible at the 1σ level between U and Q bands. With the benefit of quasi-simultaneous U and Q band observations in 2010, we have studied chromatic effects (core-shift) at the radio source cores with three different methods. The magnitudes of the core-shifts are of the same order (about 0.1 mas) for all methods. However, some discrepancies arise in the orientation of the core-shifts determined through the different methods. In some cases these discrepancies are due to insufficient signal for the method used. In others, the discrepancies reflect assumptions of the methods and could be explained by curvatures in the jets and departures from conical jets.

scholarly journals Inferring the jet parameters of active galactic nuclei using Bayesian analysis of VLBI data with a non-uniform jet model

2019 ◽  
Vol 488 (1) ◽  
pp. 939-953 ◽  
Author(s):  
Ilya N Pashchenko ◽  
Alexander V Plavin
Keyword(s):  
Bayesian Analysis ◽  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Peak Frequency ◽  
Physical Parameters ◽  
Image Deconvolution ◽  
Long Baseline ◽  
Central Engine ◽  
Electron Positron ◽  
Vlbi Data

ABSTRACTThe physical parameters of the jets of active galactic nuclei observed with Very Long Baseline Interferometry (VLBI) are usually inferred from core-shift measurements or from the flux and size measured at the peak frequency of the synchrotron spectrum. Both methods are preceded by modelling the observed VLBI jet structure with simple Gaussian templates. Here we infer the jet parameters using an inhomogeneous jet model directly, bypassing the modelling of the source structure with a Gaussian template or image deconvolution. We apply Bayesian analysis to multifrequency VLBA observations of radio galaxy NGC 315 and find that its parsec-scale jet is well described by an inhomogeneous conical model. Our results favour an electron–positron jet. We also detect a component as a part of a counter jet. Its position implies the presence of an external absorber with a steep density gradient close (r = 0.1 pc) to the central engine.

scholarly journals The Milliarcsecond Structure of Active Galactic Nuclei Observed with VLBI

1989 ◽  
Vol 134 ◽  
pp. 525-528
Author(s):  
T. J. Pearson ◽  
A. C. S. Readhead
Keyword(s):  
Active Galactic Nuclei ◽  
Very Long Baseline Interferometry ◽  
Galactic Nuclei ◽  
Radio Galaxies ◽  
Relativistic Motion ◽  
Powerful Radio ◽  
Long Baseline ◽  
Vlbi Observations ◽  
Central Engine

Very Long Baseline Interferometry at radio wavelengths is the only technique available for imaging the central few parsecs of powerful radio galaxies and quasars. VLBI observations have shown that in many nuclei radio-emitting material is collimated into a jet on a scale less than a parsec and ejected at relativistic velocities. The interpretation of the observations is complicated by the relativistic motion, however: the images are dominated by those parts of the source that are moving almost directly towards the observer, and thus amplified by relativistic aberration. Nonetheless, the VLBI images are vital for understanding the nature of the central engine, the cause of the collimation, and the physics of the jets.

Very long baseline interferometry polarization studies of quasars and active galactic nuclei

1986 ◽  
Vol 64 (4) ◽  
pp. 434-439 ◽  
Author(s):  
J. F. C. Wardle ◽  
D. H. Roberts
Keyword(s):  
Active Galactic Nuclei ◽  
Linear Polarization ◽  
Very Long Baseline Interferometry ◽  
Galactic Nuclei ◽  
Radio Sources ◽  
Long Baseline ◽  
First Results ◽  
Polarization Studies ◽  
Theoretical Issues

We present some first results of a program to map the distribution of linear polarization in compact radio sources with milliarcsecond resolution. We show first-epoch maps of 3C345 and 0735 + 178 and first- and second-epoch maps of OJ287. In general, the polarization is mainly associated with optically thin (jet) components. In the case of OJ287, polarization maps made 1 year apart are strikingly different. We also discuss some of the theoretical issues raised by these observations.

scholarly journals High-resolution radio imaging of two luminous quasars beyond redshift 4.5

2018 ◽  
Vol 618 ◽  
pp. A68 ◽  
Author(s):  
S. Frey ◽  
O. Titov ◽  
A. E. Melnikov ◽  
P. de Vicente ◽  
F. Shu
Keyword(s):  
High Resolution ◽  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Brightness Distribution ◽  
Distribution Model ◽  
Radio Structure ◽  
Long Baseline ◽  
Radio Imaging ◽  
Jet Structure ◽  
East West

Context. Radio-loud active galactic nuclei in the early Universe are rare. The quasars J0906+6930 at redshift z = 5.47 and J2102+6015 at z = 4.57 stand out from the known sample with their compact emission on milliarcsecond (mas) angular scale with high (0.1 Jy level) flux densities measured at GHz radio frequencies. This makes them ideal targets for very long baseline interferometry (VLBI) observations. Aims. By means of VLBI imaging we can reveal the inner radio structure of quasars and model their brightness distribution to better understand the geometry of the jet and the physics of the sources. Methods. We present sensitive high-resolution VLBI images of J0906+6930 and J2102+6015 at two observing frequencies, 2.3 and 8.6 GHz. The data were taken in an astrometric observing programme involving a global five-element radio telescope array. We combined the data from five different epochs from 2017 February to August. Results. For one of the highest redshift blazars known, J0906+6930, we present the first-ever VLBI image obtained at a frequency below 8 GHz. Based on our images at 2.3 and 8.6 GHz, we confirm that this source has a sharply bent helical inner jet structure within ∼3 mas from the core. The quasar J2102+6015 shows an elongated radio structure in the east–west direction within the innermost ∼2 mas that can be described with a symmetric three-component brightness distribution model at 8.6 GHz. Because of their non-pointlike mas-scale structure, these sources are not ideal as astrometric reference objects. Our results demonstrate that VLBI observing programmes conducted primarily with astrometric or geodetic goals can be utilized for astrophysical purposes as well.

scholarly journals Monitoring Of Jets in Active Galactic Nuclei with VLBA Experiments. XVIII. Kinematics and Inner Jet Evolution of Bright Radio-loud Active Galaxies

2021 ◽  
Vol 923 (1) ◽  
pp. 30
Author(s):  
M. L. Lister ◽  
D. C. Homan ◽  
K. I. Kellermann ◽  
Y. Y. Kovalev ◽  
A. B. Pushkarev ◽  
...  
Keyword(s):  
Active Galactic Nuclei ◽  
Gamma Ray ◽  
Flow Instability ◽  
Galactic Nuclei ◽  
Position Angle ◽  
Spectral Energy ◽  
Large Area ◽  
Bl Lac Objects ◽  
Long Baseline ◽  
Bona Fide

Abstract We have analyzed the parsec-scale jet kinematics of 447 bright radio-loud active active galactic nuclei (AGN), based on 15 GHz Very Long Baseline Array (VLBA) data obtained between 1994 August 31 and 2019 August 4. We present new total intensity and linear polarization maps obtained between 2017 January 1 and 2019 August 4 for 143 of these AGN. We tracked 1923 bright features for five or more epochs in 419 jets. The majority (60%) of the well-sampled jet features show either accelerated or nonradial motion. In 47 jets there is at least one nonaccelerating feature with an unusually slow apparent speed. Most of the jets show variations of 10°–50° in their inner jet position angle (PA) over time, although the overall distribution has a continuous tail out to 200°. AGN with spectral energy distributions peaked at lower frequencies tend to have more variable PAs, with BL Lac objects being less variable than quasars. The Fermi Large Area Telescope (LAT) gamma-ray-associated AGN also tend to have more variable PAs than the non-LAT AGN in our sample. We attribute these trends to smaller viewing angles for the lower spectral peaked and LAT-associated jets. We identified 13 AGN where multiple features emerge over decade-long periods at systematically increasing or decreasing PAs. Since the ejected features do not fill the entire jet cross section, this behavior is indicative of a precessing flow instability near the jet base. Although some jets show indications of oscillatory PA evolution, we claim no bona fide cases of periodicity since the fitted periods are comparable to the total VLBA time coverage.

THE PARSEC-SCALE CIRCULAR POLARIZATION OF ACTIVE GALACTIC NUCLEI AT 22 AND 15 GHZ

2008 ◽  
Vol 17 (09) ◽  
pp. 1531-1535 ◽  
Author(s):  
V. M. VITRISHCHAK ◽  
D. C. GABUZDA
Keyword(s):  
Magnetic Field ◽  
Active Galactic Nuclei ◽  
Circular Polarization ◽  
Galactic Nuclei ◽  
Core Region ◽  
Field Orientation ◽  
Magnetic Field Orientation ◽  
Long Baseline ◽  
Very Long Baseline Array ◽  
The Magnetic Field

We present the results of parsec-scale circular polarization measurements based on Very Long Baseline Array data for a number of radio-bright, core-dominated active galactic nuclei obtained simultaneously at 22 and 15 GHz. The degrees of circular polarization mc for the VLBI core region at 15 GHz are similar to values reported earlier at this wavelength, with typical values of a few tenths of a percent. The origin of this polarization is almost certainly the conversion of linear to circular polarization during the propagation of the radiation through a magnetised plasma. We find that mc is as often higher as lower at the higher frequency, for reasons that are not clear. Our results confirm the earlier finding that the sign of the circular polarization at a given observing frequency is generally consistent across epochs separated by several years or more, suggesting stability of the magnetic field orientation in the innermost jets.

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