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

TL;DR

This study analyzes EVPA rotations in blazar 3C 279, distinguishing stochastic and deterministic origins of polarization variability through extensive optical data, revealing different governing processes during flux states.

Contribution

It introduces a quantitative measure for EVPA curve smoothness and applies it to differentiate polarization variability models in 3C 279.

Findings

EVPA rotations are observed during 2008-2012 in 3C 279.

Flaring states show smooth EVPA rotations inconsistent with stochastic models.

Different polarization processes are active during low-flux and flaring states.

Abstract

Over the past few years, several occasions of 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, as they could allow one to probe the magnetic field structure in the gamma-ray emitting region of the jet. The flat-spectrum radio quasar 3C279 is one of the most prominent examples showing this behaviour. Our goal is to study the observed EVPA rotations and to distinguish between a stochastic and a deterministic origin of the polarization variability. We have combined multiple data sets of R-band photometry and optical polarimetry measurements of 3C279, 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. 3C279 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 approx. 360 degrees rotation of the EVPA in mid-2011, is not consistent with the tested stochastic processes. We conclude that during the two different optical flux states, two different processes govern the polarization variation, possibly a stochastic process during the low-brightness state and a deterministic process during the flaring activity.