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A VLBA survey of the core shift effect in AGN jets I. Evidence for dominating synchrotron opacity | | K. V. Sokolovsky
; Y. Y. Kovalev
; A. B. Pushkarev
; A. P. Lobanov
; | | Date: |
30 Mar 2011 | | Abstract: | The effect of a frequency dependent shift of the VLBI core position (known as
the "core shift") has been predicted more than three decades ago and has since
been observed in a few sources, but often within a narrow frequency range. This
effect has important astrophysical and astrometric applications. In order to
achieve a broader understanding of the core shift effect and the physics behind
it we have conducted a dedicated survey with NRAO’s Very Long Baseline Array
(VLBA). We have used the VLBA to image 20 pre-selected sources simultaneously
at nine frequencies in the 1.4-15.4 GHz range. The core position at each
frequency was measured by referencing it to a bright, optically thin feature in
the jet. A significant core shift has been successfully measured in each of the
twenty sources observed. The median value of the core shift was found to be
1.21 mas if measured between 1.4 and 15.4 GHz, and 0.24 mas between 5.0 and
15.4 GHz. The core position, r, as a function of frequency, n, is found to be
consistent with a "r proportional to 1/n" law. Such behavior is predicted by
the Blandford & Koenigl model of a purely synchrotron self-absorbed conical jet
in equipartition. No systematic deviation from unity of the power law index in
the r(n) relation has been convincingly detected. We conclude that neither
free-free absorption nor gradients in pressure and/or density in the jet itself
and in the ambient medium surrounding the jet play a significant role in the
sources observed within the 1.4-15.4 GHz frequency range. These results support
the interpretation of the parsec-scale core as a continuous Blandford-Koenigl
type jet with smooth gradients of physical properties along it. | | Source: | arXiv, 1103.6032 | | Services: | Forum | Review | PDF | Favorites | |
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