Millimeter VLBI observations of Sgr A* with KaVA and KVN

AbstractWe present recent observation results of Sgr A* at millimeter obtained with VLBI arrays in Korea and Japan.7 mm monitoring of Sgr A* is part of our AGN large project. The results at 7 epochs during 2013-2014, including high resolution maps, flux density and two-dimensional size measurements are presented. The source shows no significant variation in flux and structure related to the G2 encounter in 2014. According to recent MHD simulations by kawashimaet al., flux and magnetic field energy can be expected to increase several years after the encounter; We will keep our monitoring in order to test this prediction.Astrometric observations of Sgr A* were performed in 2015 at 7 and 3.5 millimeter simultaneously. Source-frequency phase referencing was applied and a combined ”core-shift” of Sgr A* and a nearby calibrator was measured. Future observations and analysis are necessary to determine the core-shift in each source.

Download Full-text

Millimeter-VLBI Observations of Low-luminosity Active Galactic Nuclei with Source-frequency Phase Referencing

The Astrophysical Journal ◽

10.3847/2041-8213/ac375c ◽

2021 ◽

Vol 922 (1) ◽

pp. L16

Author(s):

Wu Jiang ◽

Zhiqiang Shen ◽

Ivan Martí-Vidal ◽

Xuezheng Wang ◽

Dongrong Jiang ◽

...

Keyword(s):

Magnetic Field ◽

Field Strength ◽

Brightness Temperature ◽

Active Galactic Nuclei ◽

Event Horizon ◽

Galactic Nuclei ◽

The Core ◽

Vlbi Observations ◽

Core Shift ◽

Frequency Phase

Abstract We report millimeter-VLBI results of low-luminosity active galactic nuclei (M84 and M87) up to 88 GHz with source-frequency phase-referencing observations. We detected the weak VLBI core and obtained the first image of M84 at 88 GHz. The derived brightness temperature of the M84 core was about 7.2 × 109 K, which could serve as a lower limit because the core down to 30 Schwarzschild radii was still unresolved in our 88 GHz observations. We successfully determined the core shifts of M87 at 22–44 GHz and 44–88 GHz through the source-frequency phase-referencing technique. The jet apex of M87 could be deduced at ∼46 μas upstream of the 43 GHz core from core-shift measurements. The estimated magnetic field strength of the 88 GHz core of M87 is 4.8 ± 2.4 G, which is at the same magnitude of 1–30 G near the event horizon probed by the Event Horizon Telescope.

Download Full-text

Axisymmetric and stationary magnetic field structures in neutron star crusts under various boundary conditions

Proceedings of the International Astronomical Union ◽

10.1017/s174392131400266x ◽

2013 ◽

Vol 9 (S302) ◽

pp. 427-428

Author(s):

Kotaro Fujisawa ◽

Shota Kisaka

Keyword(s):

Magnetic Field ◽

Neutron Star ◽

Magnetic Fields ◽

Boundary Conditions ◽

Outer Boundary ◽

Energy Ratio ◽

Field Energy ◽

Various Boundary Conditions ◽

The Core ◽

Magnetic Field Energy

AbstractWe have calculated many Hall equilibrium states within the neutron star crust under various boundary conditions in order to investigate the influences of the boundary conditions clearly. We have found two important features of these solutions. First, the magnitude of the core magnetic fields affects the toroidal to total magnetic field energy ratio within the crust (Et/E). If the core magnetic fields are vanished, the crustal toroidal magnetic fields become weak and the typical energy ratio is only Et / E ~ 0.1%. If the core magnetic fields are strong, however, the crustal toroidal magnetic fields become strong and the typical ratio reaches Et / E ~ 15%. Second, the core toroidal magnetic fields and the twisted magnetosphere around the star make the size of the crustal toroidal magnetic field regions large. Therefore if the strong core magnetic fields have strong toroidal component, both strength and size of the crustal toroidal magnetic fields become large. These results show that the Hall MHD evolutions would be deeply affected by both inner and outer boundary conditions.

Download Full-text

VLBI Observations of the Radio Jet in 3C 273

Symposium - International Astronomical Union ◽

10.1017/s0074180900133844 ◽

1988 ◽

Vol 129 ◽

pp. 27-28

Author(s):

S. C. Unwin ◽

R. J. Davis

Keyword(s):

Dynamic Range ◽

Large Range ◽

High Dynamic Range ◽

No Emission ◽

The Core ◽

Vlbi Observations ◽

High Dynamic ◽

Quasar 3C 273

We present a new high dynamic range map of the quasar 3C 273, made from observations with a VLBI network of 12 telescopes. This new map at 18 cm wavelength has one of the highest dynamic ranges yet achieved with VLBI, and it shows the ‘jet’ extending to at least 180 milliarcsec, or 330 pc from the nucleus of the quasar. Strong limits can be placed on the brightness of any ‘counter-jet’ on kiloparsec-scales, as no emission is visible on the opposite side of the ‘core’. Combining with other VLBI, VLA and MERLIN maps shows that the jet is visible and continuous over a very large range of scales, from 1 pc to 40 kpc.

Download Full-text

The growth of magnetic field energy in conducting fluids

Computational Fluid and Solid Mechanics 2003 ◽

10.1016/b978-008044046-0.50245-1 ◽

2003 ◽

pp. 1004-1007

Author(s):

Philip Livermore ◽

Andrew Jackson

Keyword(s):

Magnetic Field ◽

Field Energy ◽

Magnetic Field Energy ◽

Conducting Fluids

Download Full-text

5 Years of VLBI and X-Ray Observations of NRAO 140

Symposium - International Astronomical Union ◽

10.1017/s0074180900133881 ◽

1988 ◽

Vol 129 ◽

pp. 35-36

Author(s):

Alan P. Marscher

Keyword(s):

Flux Density ◽

Low Frequency ◽

Total Flux ◽

X Ray ◽

The Core ◽

Vlbi Observations ◽

Superluminal Motion ◽

Low Frequency Variability ◽

Jet Structure

The author and his collaborators have observed NRAO 140 twice at X-ray energies and numerous times with multifrequency VLBI. The VLBI observations reveal a knotty jet structure with superluminal motion of the innermost two knots relative to the core. The VLBI core decreased by about a factor of 2 in flux density between 1980 and late 1984. The X-ray flux also declined by about the same factor during this period. Monitoring at 18 cm during periods of low-frequency variability has revealed pronounced changes in the relative brightnesses of the components of the source while the total flux density has varied by ≲ 10%.

Download Full-text

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

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731869 ◽

2018 ◽

Vol 614 ◽

pp. A74 ◽

Cited By ~ 3

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.

Download Full-text

MHD SIMULATIONS OF ACCRETION ONTO Sgr A*: QUIESCENT FLUCTUATIONS, OUTBURSTS, AND QUASIPERIODICITY

The Astrophysical Journal ◽

10.1088/0004-637x/701/1/521 ◽

2009 ◽

Vol 701 (1) ◽

pp. 521-534 ◽

Cited By ~ 42

Author(s):

Chi-kwan Chan ◽

Siming Liu ◽

Christopher L. Fryer ◽

Dimitrios Psaltis ◽

Feryal Özel ◽

...

Keyword(s):

Mhd Simulations ◽

Sgr A

Download Full-text

Space VLBI Observations of PKS 1921-293

Symposium - International Astronomical Union ◽

10.1017/s0074180900220706 ◽

2001 ◽

Vol 205 ◽

pp. 128-129

Author(s):

Zhi-Qiang Shen ◽

Philip G. Edwards ◽

Seiji Kameno ◽

Makoto Inoue ◽

James E. J. Lovell

Keyword(s):

Brightness Temperature ◽

Spectral Index ◽

Rest Frame ◽

The Core ◽

Vlbi Observations ◽

First Time

We present results of the first space VLBI observations of PKS 1921-293. An inner jet component about 1.5 mas north of the core is revealed for the first time. The compact core is partially resolved, but still has a brightness temperature (at the source rest frame) of 3.0×1012 K. A spectral index map made by combining the 1.6 GHz VSOP image with the 5.0 GHz VLBA+Y image at the first epoch is also presented.

Download Full-text