A Caltech Library Service

MOJAVE: Monitoring of Jets in Active galactic nuclei with VLBA Experiments. IX. Nuclear opacity

Pushkarev, A. B. and Hovatta, T. and Kovalev, Y. Y. and Lister, M. L. and Lobanov, A. P. and Savolainen, T. and Zensus, J. A. (2012) MOJAVE: Monitoring of Jets in Active galactic nuclei with VLBA Experiments. IX. Nuclear opacity. Astronomy and Astrophysics, 545 . Art. No. A113. ISSN 0004-6361.

PDF - Published Version
See Usage Policy.


Use this Persistent URL to link to this item:


Aims. We have investigated a frequency-dependent shift in the absolute position of the optically thick apparent origin of parsec-scale jets (“core shift” effect) to probe physical conditions in ultra-compact relativistic outflows in active galactic nuclei. Methods. We used multi-frequency Very Long Baseline Array (VLBA) observations of 191 sources carried out in 12 epochs in 2006 within the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) program. The observations were performed at 8.1, 8.4, 12.1, and 15.4 GHz. We implemented a method of determining the core shift vector based on (i) image registration by two-dimensional normalized cross-correlation and (ii) model-fitting the source brightness distribution to take into account a non-zero core component offset from the phase center. Results. The 15.4−8.1, 15.4−8.4, and 15.4−12.1 GHz core shift vectors are derived for 163 sources, and have median values of 128, 125, and 88 μas, respectively, compared to the typical measured errors of 50, 51, 35 μas. The effect occurs predominantly along the jet direction, with departures smaller than 45° from the median jet position angle in over 80% of the cases. Despite the moderate ratio of the observed frequencies (<2), core shifts significantly different from zero (>2σ) are detected for about 55% of the sources. These shifts are even better aligned with the jet direction, deviating from the latter by less than 30° in over 90% of the cases. There is an indication that the core shift decreases with increasing redshift. Magnetic fields in the jet at a distance of 1 parsec from the central black hole, calculated from the obtained core shifts, are found to be systematically stronger in quasars (median B_1 ≈ 0.9 G) than those in BL Lacs (median B_1 ≈ 0.4 G). We also constrained the absolute distance of the core from the apex of the jet at 15 GHz as well as the magnetic field strength in the 15 GHz core region.

Item Type:Article
Related URLs:
URLURL TypeDescription
Pushkarev, A. B.0000-0002-9702-2307
Hovatta, T.0000-0002-2024-8199
Kovalev, Y. Y.0000-0001-9303-3263
Lister, M. L.0000-0003-1315-3412
Savolainen, T.0000-0001-6214-1085
Additional Information:© 2012 ESO. Received: 5 March 2012. Accepted: 20 July 2012. Published online 17 September 2012. We would like to thank K. I. Kellermann, E. Ros, D. C. Homan and the rest of the MOJAVE team for the productive discussions. We thank the anonymous referee for useful comments, which helped to improve the manuscript. This research has made use of data from the MOJAVE database that is maintained by the MOJAVE team (Lister et al. 2009). The MOJAVE project is supported under National Science Foundation grant AST-0807860 and NASA Fermi grant NNX08AV67G. T.H. was supported in part by the Jenny and Antti Wihuri foundation. Y.Y.K. is partly supported by the Russian Foundation for Basic Research (project 11-02-00368), Dynasty Foundation, and the basic research program “Active processes in galactic and extragalactic objects” of the Physical Sciences Division of the Russian Academy of Sciences. Part of this work was supported by the COST Action MP0905 “Black Holes in a Violent Universe”. The VLBA is a facility of the National Science Foundation operated by the National Radio Astronomy Observatory under cooperative agreement with Associated Universities, Inc.
Funding AgencyGrant Number
Jenny and Antti Wihuri FoundationUNSPECIFIED
Russian Foundation for Basic Research11-02-00368
Dynasty FoundationUNSPECIFIED
COST ActionMP0905
Russian Academy of Sciences Physical Sciences DivisionUNSPECIFIED
Subject Keywords:galaxies: active galaxies: jets radio continuum: galaxies quasars: general
Record Number:CaltechAUTHORS:20130115-141112765
Persistent URL:
Official Citation:MOJAVE: Monitoring of Jets in Active galactic nuclei with VLBA Experiments - IX. Nuclear opacity A113 A. B. Pushkarev, T. Hovatta, Y. Y. Kovalev, M. L. Lister, A. P. Lobanov, T. Savolainen and J. A. Zensus Published online: 17 September 2012 DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:36399
Deposited By: Ruth Sustaita
Deposited On:15 Jan 2013 22:35
Last Modified:09 Mar 2020 13:19

Repository Staff Only: item control page