Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Radio astrometry with chromatic AGN core positions   Aims: The effect of frequency-dependent AGN core positions (``core-shifts'')
on radio Very Long Baseline Interferometry (VLBI) global astrometry
measurements is investigated.
  Methods: The basic equations relating to VLBI astrometry are reviewed,
including the effects of source structure. A power-law representation of
core-shifts, based on both observations and theoretical considerations of jet
conditions, is incorporated.
  Results: It is shown that, in the presence of core-shifts, phase and
group-delay astrometry measurements yield different positions. For a core
displacement from the jet base parametrized by Delta x (lambda) = k lambda^beta
group delays measure a ``reduced'' core-shift of (1-beta) Delta x (lambda). For
the astrophysically-significant case of beta = 1, group delays measure no shift
at all, giving the position of the jet base. At 8.4 GHz an estimated typical
offset between phase and group-delay positions of ~170 uas is smaller than the
current ~250 uas precision of group-delay positions of the sources used to
define the ICRF; however, this effect must be taken into account for future
measurements planned with improved accuracy when comparing with optical
positions of AGN to be obtained with the GAIA mission.

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The categories of this paper are: dg-ga
31
The categories of this paper are: ao-sci
37
The categories of this paper are: supr-con
8
The categories of this paper are: hep-ex
12
The categories of this paper are: nucl-th
33
The categories of this paper are: plasm-ph
10
The categories of this paper are: q-bio
27
The categories of this paper are: comp-gas
35
The categories of this paper are: mtrl-th
34
The categories of this paper are: chem-ph
5
The categories of this paper are: gr-qc
2
The categories of this paper are: math
0
The categories of this paper are: hep-ph
32
The categories of this paper are: atom-ph
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The categories of this paper are: adap-org
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The categories of this paper are: q-fin