Volume 505, Number 1, October I 2009
|Page(s)||L1 - L4|
|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: firstname.lastname@example.org
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
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.