A VLBA survey of the core shift effect in AGN jets - I. Evidence of dominating synchrotron opacity

By K. Sokolovsky, Y. Kovalev, A. Pushkarev, A. Lobanov. 2011

144 citations recorded by the source corpus.

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. 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). 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. 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, r, as a function of frequency, n, is found to be consistent with an r n^-1 law. This 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.

Subfields: Aerospace Engineering, Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/151c2dc0e8256d6513503303681d064db67f3bd6

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Aerospace Engineering

1. M. J. Reid
2. M. Honma

In Astronomy and Astrophysics

1. Y. Kovalev
2. A. Pushkarev
3. T. Savolainen
4. M. Lister
5. A. Lobanov
6. J. Zensus
7. T. Hovatta
8. M. Honma

In Nuclear and High Energy Physics

1. Y. Kovalev
2. A. Pushkarev
3. T. Savolainen
4. M. Lister
5. A. Lobanov
6. J. Zensus
7. T. Hovatta
8. M. J. Reid

Citations in the corpus

• Microarcsecond Radio Astrometry (2013, background): “…black hole of M 87 (see Figure 4) and located the black hole relative to the jet emission with an accuracy of ∼ 10Rsch. Kovalev et al. (2008) and Sokolovsky et al. (2011) have confirmed that frequency-dependent core-shifts, such as observed in M 87, are common in large samples of AGNs with…”
• Detection of significant cm to sub-mm band radio and γ-ray correlated variability in Fermi bright blazars (2014, background): “…radio opacity is also commonly observed as frequency-dependent “core shifts” in multi-frequency VLBI images (e.g. Lobanov 1998; Kovalev et al. 2008; Sokolovsky et al. 2011) with the core being identified as the most compact jet feature near the apparent base of the jet, and the surface at which…”
• MOJAVE: Monitoring of Jets in Active galactic nuclei with VLBA Experiments (2012, methodology): “Butif the 15.4-8.1 GHz core shifts for the 20-source sample are calculated from the fitted hyperbolas (Sokolovsky et al. 2011), which provide more accurate core shift values, the corresponding median yields 0.127 mas, which agrees well with the median of our sample.”