Issue |
A&A
Volume 505, Number 1, October I 2009
|
|
---|---|---|
Page(s) | L1 - L4 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/200912846 | |
Published online | 03 September 2009 |
Letter to the Editor
Radio astrometry with chromatic AGN core positions
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: porcas@mpifr-bonn.mpg.de
Received:
8
July
2009
Accepted:
25
August
2009
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 , group delays measure a “reduced” core-shift of
. For the astrophysically-significant case of β = 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 μas is smaller than the current ~250 μas 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.
Key words: astrometry / reference systems / galaxies: jets / techniques: interferometric
© ESO, 2009
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