Polarization angle swings in blazars: The case of 3C 279^⋆

¹ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
e-mail: sebastian.kiehlmann@aalto.fi
² Aalto University Department of Radio Science and Engineering, PL 13000, 00076 Aalto, Finland
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel, 69, 53121 Bonn, Germany
⁴ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁵ Astronomical Institute, St. Petersburg State University, Universitetskij Pr. 28, Petrodvorets, 198504 St. Petersburg, Russia
⁶ Astro Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia
⁷ Sternberg Astronomical Institute, M.V.Lomonosov Moscow State University, Universiteskij prosp. 13, 119991 Moscow, Russia
⁸ Institute of Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens Vas. Pavlou & I. Metaxa, 15 236 Penteli, Greece
⁹ Department of Physics and Astronomy, University of New Mexico, Albuquerque NM, 87131, USA
¹⁰ Main (Pulkovo) Astronomical Observatory of RAS, Pulkovskoye shosse, 60, 196140 St. Petersburg, Russia
¹¹ Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain
¹² Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, 739-8526 Hiroshima, Japan
¹³ Instituto de Astronomía,Universidad Nacional Autónoma de México, 04510 México DF, México
¹⁴ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
¹⁵ Foundation for Research and Technology – Hellas, IESL, Voutes, 71110 Heraklion, Greece
¹⁶ Department of Physics and Institute for Plasma Physics, University of Crete, 71003 Heraklion, Greece
¹⁷ Crimean Astrophysical Observatory, P/O Nauchny, 98409 Crimea, Russia
¹⁸ Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz, 369167 Karachaevo-Cherkesia, Russia
¹⁹ Southern Station of the Sternberg Astronomical Institute, Moscow M.V. Lomonosov State University, P/O Nauchny, 298409 Crimea, Russia
²⁰ Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, 739-8526 Hiroshima, Japan
²¹ ZAH, Landessternwarte Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
²² Instituto de Astronomía, Universidad Nacional Autónoma de México, 22860 Ensenada BC, México
²³ Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
²⁴ Inst. of Solar-Terrestrial Physics, Lermontov st. 126a, Irkutsk p/o box 291, 664033, Russia
²⁵ Center for Backyard Astrophysics – New Mexico, PO Box 1351 Cloudcroft, NM 88317, USA
²⁶ Engelhardt Astronomical Observatory, Kazan Federal University, Tatarstan, Russia
²⁷ Department of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
²⁸ Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo, 5-1-5 Kashiwa-no-Ha, 277-8583 Kashiwa City Chiba, Japan
²⁹ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
³⁰ Department of Physics, University of Colorado Denver, CO, USA
³¹ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
³² Lowell Observatory, Flagstaff, AZ 86001, USA

Received: 10 November 2015
Accepted: 29 February 2016

Abstract

Context. Over the past few years, on several occasions, large, continuous rotations of the electric vector position angle (EVPA) of linearly polarized optical emission from blazars have been reported. These events are often coincident with high energy γ-ray flares and they have attracted considerable attention, since they could allow us to probe the magnetic field structure in the γ-ray emitting region of the jet. The flat-spectrum radio quasar 3C 279 is one of the most prominent examples showing this behaviour.

Aims. Our goal is to study the observed EVPA rotations and to distinguish between a stochastic and a deterministic origin of the polarization variability.

Methods. We have combined multiple data sets of R-band photometry and optical polarimetry measurements of 3C 279, yielding exceptionally well-sampled flux density and polarization curves that cover a period of 2008–2012. Several large EVPA rotations are identified in the data. We introduce a quantitative measure for the EVPA curve smoothness, which is then used to test a set of simple random walk polarization variability models against the data.

Results. 3C 279 shows different polarization variation characteristics during an optical low-flux state and a flaring state. The polarization variation during the flaring state, especially the smooth ~360° rotation of the EVPA in mid-2011, is not consistent with the tested stochastic processes.

Conclusions. We conclude that, during the two different optical flux states, two different processes govern polarization variation, which is possibly a stochastic process during the low-brightness state and a deterministic process during the flaring activity.