OH MASER SOURCES IN W49N: PROBING MAGNETIC FIELD AND DIFFERENTIAL ANISOTROPIC SCATTERING WITH ZEEMAN PAIRS USING THE VERY LONG BASELINE ARRAY

Extragalactic peaked-spectrum radio sources are thought to be the progenitors of larger, radio-loud active galactic nuclei (AGN). Synchrotron self-absorption (SSA) has often been identified as the cause of their spectral peak. The identification of new megahertz-peaked spectrum sources from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey provides an opportunity to test how radio sources with spectral peaks below 1 GHz fit within this evolutionary picture. We observed six peaked-spectrum sources selected from the GLEAM survey, three that have spectral characteristics which violate SSA and three that have spectral peaks below 230 MHz, with the Very Long Baseline Array at 1.55 and 4.96 GHz. We present milliarcsecond resolution images of each source and constrain their morphology, linear size, luminosity, and magnetic field strength. Of the sources that are resolved by our study, the sources that violate SSA appear to be compact doubles, while the sources with peak frequencies below 230 MHz have core-jet features. We find that all of our sources are smaller than expected from SSA by factors of ≳20. We also find that component magnetic field strengths calculated from SSA are likely inaccurate, differing by factors of ≳5 from equipartition estimates. The calculated equipartition magnetic field strengths more closely resemble estimates from previously studied gigahertz-peaked spectrum sources. Exploring a model of the interaction between jets and the interstellar medium, we demonstrate that free-free absorption (FFA) can accurately describe the linear sizes and peak frequencies of our sources. Our findings support the theory that there is a fraction of peaked-spectrum sources whose spectral peaks are best modeled by FFA, implying our understanding of the early stages of radio AGN is incomplete.

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The jets of AGN as giant coaxial cables

Astronomy and Astrophysics ◽

10.1051/0004-6361/201732136 ◽

2018 ◽

Vol 612 ◽

pp. A67 ◽

Cited By ~ 7

Author(s):

Denise C. Gabuzda ◽

Matt Nagle ◽

Naomi Roche

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Faraday Rotation ◽

Toroidal Magnetic Field ◽

Systematic Change ◽

Physical Mechanisms ◽

Preferred Direction ◽

Long Baseline ◽

Rotation Measure ◽

Very Long Baseline Array

Context. The currents carried by the jets of active galactic nuclei (AGNs) can be probed using maps of the Faraday rotation measure (RM), since a jet current will be accompanied by a toroidal magnetic field, which will give rise to a systematic change in the RM across the jet. Aims. The aim of this study is to identify new AGNs displaying statistically significant transverse RM gradients across their parsec-scale jets, in order to determine how often helical magnetic fields occur in AGN jets, and to look for overall patterns in the implied directions for the toroidal field components and jet currents. Methods. We have carried out new analyses of Faraday RM maps derived from previously published 8.1, 8.4, 12.1 and 15.3 GHz data obtained in 2006 on the NRAO Very Long Baseline Array (VLBA). In a number of key ways, our procedures were identical to those of the original authors, but the new imaging and analysis differs from the original methods in several ways: the technique used to match the resolutions at the different frequencies, limits on the widths spanned by the RM gradients analyzed, treatment of core-region RM gradients, approach to estimation of the significances of the gradients analyzed, and inclusion of a supplementary analysis using circular beams with areas equal to those of the corresponding elliptical naturally weighted beams. Results. This new analysis has substantially increased the number of AGNs known to display transverse RM gradients that may reflect the presence of a toroidal magnetic-field component. The collected data on parsec and kiloparsec scales indicate that the current typically flows inward along the jet axis and outward in a more extended region surrounding the jet, typical to the current structure of a co-axial cable, accompanied by a self-consistent system of nested helical magnetic fields, whose toroidal components give rise to the observed transverse Faraday rotation gradients. Conclusions. The new results presented here make it possible for the first time to conclusively demonstrate the existence of a preferred direction for the toroidal magnetic-field components – and therefore of the currents – of AGN jets. Discerning the origin of this current-field system is of cardinal importance for understanding the physical mechanisms leading to the formation of the intrinsic jet magnetic field, which likely plays an important role in the propagation and collimation of the jets; one possibility is the action of a “cosmic battery”.

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

International Journal of Modern Physics D ◽

10.1142/s021827180801311x ◽

2008 ◽

Vol 17 (09) ◽

pp. 1531-1535 ◽

Cited By ~ 1

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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OH Maser sources in W49N: probing differential anisotropic scattering with Zeeman pairs

Proceedings of the International Astronomical Union ◽

10.1017/s1743921312007545 ◽

2012 ◽

Vol 8 (S287) ◽

pp. 470-474

Author(s):

Avinash A. Deshpande ◽

W. M. Goss ◽

J. E. Mendoza-Torres

Keyword(s):

Magnetic Field ◽

Galactic Plane ◽

Position Angle ◽

Anisotropic Scattering ◽

Local Magnetic Field ◽

High Angular Resolution ◽

Apparent Velocity ◽

Image Position ◽

Electron Density Irregularities ◽

Maser Sources

AbstractOur analysis of a VLBA 12-hour synthesis observations of the OH masers in W49N has provided detailed high angular-resolution images of the maser sources, at 1612, 1665 and 1667 MHz. The images, of several dozens of spots, reveal anisotropic scatter broadening; with typical sizes of a few tens of milli-arc-seconds and axial ratios between 1.5 to 3. The image position angles oriented perpendicular to the galactic plane are interpreted in terms of elongation of electron-density irregularities parallel to the galactic plane, due to a similarly aligned local magnetic field. However, we find the apparent angular sizes on the average a factor of 2.5 less than those reported by Desai et al., indicating significantly less scattering than inferred earlier. The average position angle of the scattered broadened images is also seen to deviate significantly (by about 10 degrees) from that implied by the magnetic field in the Galactic plane. More intriguingly, for a few Zeeman pairs in our set, we find significant differences in the scatter broadened images for the two hands of polarization, even when apparent velocity separation is less than 0.1 km/s. Here we present the details of our observations and analysis, and discuss the interesting implications of our results for the intervening anisotropic magneto-ionic medium, as well as a comparison with the expectations based on earlier work.

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The First VERA Astrometry Catalog

Publications of the Astronomical Society of Japan ◽

10.1093/pasj/psaa018 ◽

2020 ◽

Vol 72 (4) ◽

Cited By ~ 6

Author(s):

Tomoya Hirota ◽

◽

Takumi Nagayama ◽

Mareki Honma ◽

Yuuki Adachi ◽

...

Keyword(s):

Spiral Structure ◽

Galactic Center ◽

Rapid Variation ◽

Error Sources ◽

Long Baseline ◽

Circular Rotation ◽

Motion Measurements ◽

Very Long Baseline Array ◽

Source Structure ◽

Maser Sources

Abstract We present the first astrometry catalog from the Japanese VLBI (very long baseline interferometer) project VERA (VLBI Exploration of Radio Astrometry). We have compiled all the astrometry results from VERA, providing accurate trigonometric-annual-parallax and proper-motion measurements. In total, 99 maser sources are listed in the VERA catalog. Among them, 21 maser sources are newly reported, while the rest of the 78 sources are referred to in previously published results or those in preparation for forthcoming papers. The accuracy in the VERA astrometry is revisited and compared with that from the other VLBI astrometry projects such as BeSSeL (The Bar and Spiral Structure Legacy) Survey and GOBELINS (the Gould’s Belt Distances Survey) with the VLBA (Very Long Baseline Array). We have confirmed that most of the astrometry results are consistent with each other, and the largest error sources are due to source structure of the maser features and their rapid variation, along with the systematic calibration errors and different analysis methods. Combined with the BeSSeL results, we estimate the up-to-date fundamental Galactic parameters of $R_{0}=7.92\pm 0.16_{\rm {stat.}}\pm 0.3_{\rm {sys.}}\:$kpc and $\Omega _{\odot }=30.17\pm 0.27_{\rm {stat.}}\pm 0.3_{\rm {sys.}}\:$km$\:$s$^{-1}\:$kpc$^{-1}$, where $R_{0}$ and $\Omega _{\odot }$ are the distance from the Sun to the Galactic center and the Sun’s angular velocity of the Galactic circular rotation, respectively.

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Water Maser Emission Around Low/Intermediate Mass Evolved Stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921312006680 ◽

2012 ◽

Vol 8 (S287) ◽

pp. 79-80

Author(s):

M. L. Leal-Ferreira ◽

W. H. T. Vlemmings ◽

P. J. Diamond ◽

A. Kemball ◽

N. Amiri ◽

...

Keyword(s):

Magnetic Field ◽

Circular Polarization ◽

Linear Polarization ◽

Central Star ◽

Intermediate Mass ◽

Evolved Stars ◽

Long Baseline ◽

Regular Variable ◽

Very Long Baseline Array ◽

Water Maser

AbstractWe present results of Very Long Baseline Array (VLBA) polarimetric 22 GHz H2O maser observations of a number of low/intermediate mass evolved stars. We observed 3 Miras (Ap Lyn, IK Tau and IRC+60370), 1 semi-regular variable (RT Vir) and 1 pPN (OH231.8+4.2). Circular polarization is detected in the H2O maser region of OH231.8+4.2 and we infer a magnetic field of |B||| = ~45 mG. This implies an extrapolated magnetic field of ~2.5 G on the surface of the central star. The preliminary results on RT Vir and IRC+60370 also indicate the first detection of weak H2O maser linear polarization.

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PROBING HELICAL MAGNETIC FIELDS IN AGN BY ROTATION MEASURE GRADIENTS STUDIES

International Journal of Modern Physics Conference Series ◽

10.1142/s2010194512004758 ◽

2012 ◽

Vol 08 ◽

pp. 303-306

Author(s):

JUAN C. ALGABA

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Field Strength ◽

Pitch Angle ◽

Line Of Sight ◽

Long Baseline ◽

Rotation Measure ◽

Very Long Baseline Array ◽

Bl Lacs ◽

The Magnetic Field

One of the tools that can provide evidence about the existence of helical magnetic fields in AGN is the observation of rotation measure gradients across the jet. Such observations have been previously made successfully, proving that such gradients are far from being rare, but common and typically persistent over several years, although some of them may show a reversal in the direction along the jet. Further studies of rotation measure gradients can help us in our understanding of the magnetic field properties and structure in the base of the jets. We studied Very Long Baseline Array (VLBA) polarimetric observations of 8 sources consistent of some quasars and BL Lacs at 12, 15, 22, 24 and 43 GHz and we find that all but two sources show indications of rotation measure gradients, either parallel or perpendicular to the jet. We interpret gradients perpendicular to the jet as indications of the change of the line of sight of the magnetic field due to its helicity, and gradients parallel to the jet as the decrease of magnetic field strength and/or electron density as we move along the jet. When comparing our results with the literature, we find temptative evidence of a rotation measure gradient flip, which can be explained as a change of the pitch angle or jet bending.

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VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology

Proceedings of the International Astronomical Union ◽

10.1017/s1743921312006631 ◽

2012 ◽

Vol 8 (S287) ◽

pp. 54-58

Author(s):

N. Amiri ◽

W. H. T. Vlemmings ◽

A. J. Kemball ◽

H. J. van Langevelde

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Circular Polarization ◽

Linear Polarization ◽

Central Star ◽

Dipole Field ◽

Long Baseline ◽

Very Long Baseline Array ◽

Circumstellar Environment

AbstractWe report on Very Long Baseline Array SiO maser observations of the OH/IR star OH 44.8 - 2.3. The observations show that the maser features form a ring located at a distance of 5.4 AU around the central star. The masers show high fractional linear polarization up to 100%. The polarization vectors are consistent with a dipole field morphology. Additionally, we report a tentative detection of circular polarization of 7% for the brightest maser feature. This indicates a magnetic field of 1.5 ± 0.3 G. The SiO masers and the 1612 MHz OH masers suggest a mildly preferred outflow direction in the circumstellar environment of this star. The observed polarization is consistent with magnetic field structures along the preferred outflow direction. This could indicate the possible role of the magnetic fields in shaping the circumstellar environment of this object.

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A radio parallax to the black hole X-ray binary MAXI J1820+070

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slaa010 ◽

2020 ◽

Vol 493 (1) ◽

pp. L81-L86 ◽

Cited By ~ 12

Author(s):

P Atri ◽

J C A Miller-Jones ◽

A Bahramian ◽

R M Plotkin ◽

A T Deller ◽

...

Keyword(s):

Black Hole ◽

Inclination Angle ◽

Precise Measurement ◽

Very Long Baseline Interferometry ◽

X Ray ◽

Long Baseline ◽

Model Independent ◽

Jet Velocity ◽

Very Long Baseline Array

ABSTRACT Using the Very Long Baseline Array and the European Very Long Baseline Interferometry Network, we have made a precise measurement of the radio parallax of the black hole X-ray binary MAXI J1820+070, providing a model-independent distance to the source. Our parallax measurement of (0.348 ± 0.033) mas for MAXI J1820+070 translates to a distance of (2.96 ± 0.33) kpc. This distance implies that the source reached (15 ± 3) per cent of the Eddington luminosity at the peak of its outburst. Further, we use this distance to refine previous estimates of the jet inclination angle, jet velocity, and the mass of the black hole in MAXI J1820+070 to be (63 ± 3)°, (0.89 ± 0.09) c, and (9.2 ± 1.3) M⊙, respectively.

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