VLBA polarimetric monitoring of 3C 111

¹ Dr. Remeis-Sternwarte & Erlangen Centre for Astroparticle Physics, Universität Erlangen-Nürnberg, Sternwartstrasse 7, 96049 Bamberg, Germany
e-mail: t.beuchert@uva.nl
² Lehrstuhl für Astronomie, Universität Würzburg, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
³ Departament d’Astronomia i Astrofísica, Universitat de València C/ Dr. Moliner 50, 46100 Burjassot, València, Spain
⁴ Observatori Astronòmic, Universitat de València, Parc Científic, C. Catedrático José Beltrán 2, 46980 Paterna, València, Spain
⁵ Max-Planck-Institut für extraterrestrische Physik (MPE), PO 1312, 85741 Garching, Germany
⁶ Netherlands Institute for Radio Astronomy (ASTRON), PO Box 2, 7990 AA Dwingeloo, The Netherlands
⁷ Instituto de Astrofísica de Andalucía-CSIC, Apartado 3004, 18080 Granada, Spain
⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
¹⁰ Department of Physics, Denison University, 100 W College St., Granville, OH 43023, USA
¹¹ Astro Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia
¹² Moscow Institute of Physics and Technology, Dolgoprudny, Institutsky per., 9, 141700 Moscow region, Russia
¹³ Department of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907, USA
¹⁴ Anton Pannekoek Institute for Astronomy, PO Box 94249, 1090 GE Amsterdam, The Netherlands
¹⁵ Crimean Astrophysical Observatory, 98409 Nauchny, Crimea
¹⁶ Aalto University Department of Electronics and Nanoengineering, PL 15500, 00076 Aalto, Finland
¹⁷ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
¹⁸ Instituto de Radio Astronomía Milimétrica, Avenida Divina Pastora, 7, Local 20, 18012 Granada, Spain

Received: 14 September 2017
Accepted: 30 October 2017

Abstract

Context. While studies of large samples of jets of active galactic nuclei (AGN) are important in order to establish a global picture, dedicated single-source studies are an invaluable tool for probing crucial processes within jets on parsec scales. These processes involve in particular the formation and geometry of the jet magnetic field as well as the flow itself.

Aims. We aim to better understand the dynamics within relativistic magneto-hydrodynamical flows in the extreme environment and close vicinity of supermassive black holes.

Methods. We analyze the peculiar radio galaxy 3C 111, for which long-term polarimetric observations are available. We make use of the high spatial resolution of the VLBA network and the MOJAVE monitoring program, which provides high data quality also for single sources and allows us to study jet dynamics on parsec scales in full polarization with an evenly sampled time-domain. While electric vectors can probe the underlying magnetic field, other properties of the jet such as the variable (polarized) flux density, feature size, and brightness temperature, can give valuable insights into the flow itself. We complement the VLBA data with data from the IRAM 30-m Telescope as well as the SMA.

Results. We observe a complex evolution of the polarized jet. The electric vector position angles (EVPAs) of features traveling down the jet perform a large rotation of ≳180∘ across a distance of about 20 pc. As opposed to this smooth swing, the EVPAs are strongly variable within the first parsecs of the jet. We find an overall tendency towards transverse EVPAs across the jet with a local anomaly of aligned vectors in between. The polarized flux density increases rapidly at that distance and eventually saturates towards the outermost observable regions. The transverse extent of the flow suddenly decreases simultaneously to a jump in brightness temperature around where we observe the EVPAs to turn into alignment with the jet flow. Also the gradient of the feature size and particle density with distance steepens significantly at that region.

Conclusions. We interpret the propagating polarized features as shocks and the observed local anomalies as the interaction of these shocks with a localized recollimation shock of the underlying flow. Together with a sheared magnetic field, this shock-shock interaction can explain the large rotation of the EVPA. The superimposed variability of the EVPAs close to the core is likely related to a clumpy Faraday screen, which also contributes significantly to the observed EVPA rotation in that region.