Radio astrometry with chromatic AGN core positions

Abstract

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.

Accepted for publication in Astronomy & Astrophysics, 4 pages

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