180${}^{\circ }$ Rotations in the Polarization Angle for Blazars

Abstract

Rotations of the electric vector position angle (EVPA) in blazars are often close to an integral multiple of 180${}^{\circ }$. There are multiple examples of this in the literature, and our analysis here, of the optical polarization data from the RoboPol monitoring program, strengthens the evidence by showing that $n\pi$ rotations occur more frequently than expected by chance. We explain this with a model consisting of two polarized emission components: a "jet" that is constant in time, and a "burst" that is variable. The EVPA of the combination is $\mathrm{EVP}{\mathrm{A}}_{\mathrm{jet}}$ at both the beginning and the end of the burst, so the net rotation across the burst must be $n\pi$. Examples are analyzed on the Stokes plane, where the winding number for the Stokes vector of the combination gives the value of $n$. The main conclusion is that the EVPA rotation can be much larger than the physical rotation of the emission region around the axis of the jet, but this requires the EVPAs of the jet and the burst to be nearly orthogonal. A shock-in-jet calculation by Zhang et al. can provide a physical model for our toy model, and in addition automatically gives the needed orthogonality. The model is illustrated with data on OJ287 published by Myserlis et al., and we suggest that the large rapid EVPA rotation seen there might be a phase effect and not representative of a physical rotation.

Accepted for publication in Astronomy & Astrophysics

References in corpus (10)

• A change in the optical polarization associated with a gamma-ray flare in the blazar 3C 279
• Polarization Swings Reveal Magnetic Energy Dissipation in Blazars
• Optical polarization of high-energy BL Lac objects
• RoboPol: Do optical polarization rotations occur in all blazars?
• Optical EVPA rotations in blazars: testing a stochastic variability model with RoboPol data
• The Outburst of the Blazar S40954+658 in March-April 2011
• A peculiar multi-wavelength flare in the Blazar 3C 454.3
• Linear Polarization Properties of Parsec-Scale AGN~Jets
• A Magnetohydrodynamic Model of The M87 Jet. II. Self-consistent Quad-shock Jet Model for Optical Relativistic Motions and Particle Acceleration
• Multiwavelength flaring activity of PKS1510-089

Cited by in corpus (7)

• Two Flares with One Shock: the Interesting Case of 3C 454.3
• The Time-Dependent Distribution of Optical Polarization Angle Changes in Blazars
• Repeated pattern of gamma-ray flares in the lightcurve of the blazar 3C 279
• IXPE Observation of the Low-Synchrotron Peaked Blazar S4 0954+65 During An Optical-X-ray Flare
• MOJAVE XXI. Decade-long linear polarization variability in AGN jets at parsec scales
• The Method of Searching for Rotations of the Polarization Position Angle of Quasars
• The polarisation behaviour of OJ 287 viewed through radio, millimetre and optical observations between 2015 and 2017