Polarization angle swings in blazars: the case of 3C 279
Kiehlmann, S. et. al. ; 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 gamma-ray flares and they have attracted considerable attention, since they could allow us to probe the magnetic field structure in the gamma-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 similar to 360 degrees 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. ; S.K. was supported for this research through a stipend from the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Max Planck Institute for Radio Astronomy in cooperation with the Universities of Bonn and Cologne. T.S. was partly supported by the Academy of Finland project 274477. The research at Boston University was partly funded by NASA Fermi GI grant NNX11AQ03G. K.V.S. is partly supported by the Russian Foundation for Basic Research grants 13-02-12103 and 14-02-31789. N.G.B. was supported by the RFBR grant 12-02-01237a. E.B., M.S. and D.H. thank financial support from UNAM DGAPA-PAPIIT through grant IN116211-3. I. A. acknowledges support by a Ramon y Cajal grant of the Spanish Ministry of Economy and Competitiveness (MINECO). The research at the IAA-CSIC and the MAPCAT program are supported by the Spanish Ministry of Economy and Competitiveness and the Regional Government of Andalucia (Spain) through grants AYA2010-14844, AYA2013-40825-P, and P09-FQM-4784. The Calar Alto Observatory is jointly operated by the Max-Planck-Institut fur Astronomie and the Instituto de Astrofisica de Andalucia-CSIC. Data from the Steward Observatory spectropolarimetric monitoring project were used. This program is supported by Fermi Guest Investigator grants NNX08AW56G, NNX09AU10G, NNX12AO93G, and NNX14AQ58G. St. Petersburg University team acknowledges support from Russian RFBR grant 15-02-00949 and St. Petersburg University research grant 6.38.335.2015. The Abastumani team acknowledges financial support of the project FR/638/6-320/12 by the Shota Rustaveli National Science Foundation under contract 31/77. ; Peer reviewed