Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Gas Around Active Galactic Nuclei and New Phase Calibration Strategies
  for High-Frequency VLBI   We have a project to measure magnetic fields in the obscuring tori of AGNs.
We observed five free-free absorbed jets in radio galaxies with polarimetric
VLBI at 15 GHz to determine Faraday rotation measures. Surprisingly, all
sources are unpolarized, putting severe constraints on the degree of magnetic
field turbulence and the gas distribution in these objects. We have further
carried out multi-frequency VLBI monitoring of the Seyfert 2 galaxy NGC 3079
and find unusually steep and inverted spectra in the radio components, and
derive general properties of Seyfert galaxies using published VLBI observations
of Seyfert galaxies.
  We have developed a new phase-referencing technique for high frequency VLBI
observations with the VLBA. Instead of inserting short scans on a calibrator
into the target source observations, the target source is continuously observed
while rapidly switching between the target frequency and a lower reference
frequency. We demonstrate that the technique allows phase calibration almost
reaching the thermal noise limit and present the first detection of the AGN in
NGC 4261 at 86 GHz. This is the weakest source ever detected with VLBI at that
frequency.

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The categories of this paper are: quant-ph
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The categories of this paper are: math-ph
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The categories of this paper are: supr-con
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The categories of this paper are: physics
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The categories of this paper are: dg-ga
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The categories of this paper are: stat
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The categories of this paper are: cond-mat
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The categories of this paper are: solv-int
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The categories of this paper are: ao-sci
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The categories of this paper are: mtrl-th
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The categories of this paper are: cs
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The categories of this paper are: chem-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: q-bio
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The categories of this paper are: alg-geom