Volume 532, August 2011
|Number of page(s)||31|
|Published online||19 July 2011|
A VLBA survey of the core shift effect in AGN jets
I. Evidence of dominating synchrotron opacity⋆
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
2 Astro Space Center of Lebedev Physical Institute, Profsoyuznaya Str. 84/32, 117997 Moscow, Russia
3 Pulkovo Astronomical Observatory, Pulkovskoe Chaussee 65/1, 196140 St. Petersburg, Russia
4 Crimean Astrophysical Observatory, 98409 Nauchny, Crimea, Ukraine
Received: 4 November 2010
Accepted: 29 March 2011
Context. The effect of a frequency dependent shift of the VLBI core position (known as the “core shift”) was 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.
Aims. To achieve a broader understanding of the core shift effect and the physics behind it, we conducted a dedicated survey with NRAO’s Very Long Baseline Array (VLBA).
Methods. We 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.
Results. A significant core shift has been successfully measured in each of the twenty sources observed. The median value of the core shift is 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, rc, as a function of frequency, ν, is found to be consistent with an rc ∝ ν-1 law. This behavior is predicted by the Blandford & Königl model of a purely synchrotron self-absorbed conical jet in equipartition. No systematic deviation from unity of the power law index in the rc(ν) relation has been convincingly detected.
Conclusions. 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-Königl type jet with smooth gradients of physical properties along it.
Key words: galaxies: active / galaxies: jets / quasars: general / radio continuum: general
Full Fig. 1 is available in electronic form at http://www.aanda.org
© ESO, 2011
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