Opacity in compact extragalactic radio sources and its effect on astrophysical and astrometric studies
By Y. Kovalev, A. Lobanov, A. Pushkarev, J. Zensus. 2008
154 citations recorded by the source corpus.
Context. The apparent position of the “core” in a parsec-scale radio jet (a compact, bright emitting region at the narrow end of the jet) depends on the observing frequency, owing to synchrotron self-absorption and external absorption. While providing a tool probing physical conditions in the vicinity of the core, this dependency poses problems for astrometric studies using compact radio sources. Aims. We investigated the frequency-dependent shift in the positions of the cores ( core shift ) observed with very long baseline interferometry (VLBI) in parsec-scale jets. We discuss related physics, as well as its effect on radio astrometry and the connection between radio and optical positions of astrometric reference objects. Methods. We searched for the core shift in a sample of 277 radio sources imaged at 2.3 GHz (13 cm) and 8.6 GHz (4 cm) frequency bands using VLBI observations made in 2002 and 2003. The core shift was measured by referencing the core position to optically thin jet features whose positions are not expected to change with frequency. Results. We present here results for 29 selected active galactic nuclei (AGN) with bright distinct VLBI jet features that can be used in differential measurements and that allow robust measurements of the shift to be made. In these AGN, the magnitude of the measured core shift between 2.3 and 8.6 GHz reaches 1.4 mas, with a median value for the sample of 0.44 mas. Nuclear flares result in temporal variability of the shift. Conclusions. An average shift between the radio (4 cm) and optical (6000 A) bands is estimated to be approximately 0.1 mas, and it should be taken into account in order to provide the required accuracy of the radio-optical reference frame connection. This can be accomplished with multi-frequency VLBI measurements yielding estimates of the core shift in the sources used for the radio reference frame and radio-optical position alignment.
Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics
View Paper: https://www.semanticscholar.org/paper/b4fc4fd8f08778be88c0716d8bb978746dbf3521
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Citations in the corpus
- Detection of significant cm to sub-mm band radio and γ-ray correlated variability in Fermi bright blazars (2014, background): “Furthermore, 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…”
- Microarcsecond Radio Astrometry (2013, background): “…1979) for the super-massive 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…”
- Magnetic field strength and spectral distribution of six parsec-scale active galactic nuclei jets (2009, methodology): “To calculate the magnetic field strength with distance from the base of the jet, we combine the analysis of L98 and Hirotani (2005).”
- MOJAVE: Monitoring of Jets in Active galactic nuclei with VLBA Experiments (2012, methodology): “In all cases, theimag shifts IS, obtained with NCC, were verified by visually inspecting the corresponding spectral index images before and after the alignment, because the latter are extremely sensitive to the (in)accurate image alignment, as was shown by Kovalev et al. (2008).”
- A VLBA survey of the core shift effect in AGN jets - I. Evidence of dominating synchrotron opacity (2011, methodology): “The median S/X band core shift value of 0.69 mas obtained for our sample in this study may be compared to the median value of 0.44 mas from Kovalev et al. (2008) for a sample of 29 sources.”