Variability and evolution of the optical polarization of a sample of gamma-ray blazars

Abstract

We present a polarization variability analysis of a sample of 26 $\gamma$-ray blazars monitored by the Steward Observatory between 2008 and 2018 in the optical band. We investigate the properties and long-term variability of their optical polarization, searching for differences between blazar types. We observe that BL Lac objects are typically less polarized and less variable than flat spectrum radio quasars (FSRQs). Moreover, BL Lacs display a distribution of their polarization angle typically oriented in a preferential direction, contrary to the rather random distribution of FSRQs. For the latter blazar type, as well as those sources showing a bright stellar emission, we take into account the depolarizing effect introduced by the broad line region and the host galaxy on the measured polarization degree. In this sample we also observe that BL Lacs present an uncorrelated evolution of the flux and the polarization. Contrary, FSRQs show a correlation before the depolarization correction, that is lost however after considering this effect. In addition, we study the behaviour of the polarization angle, searching for angle rotations in its long-term evolution. We derive that the FSRQs studied here show rotations more frequently than BL Lac objects by a factor $\sim$1.5. During these periods we also observe a systematic decrease of the polarization fraction, as well as a marginal flux increase, not significant however to connect rotations with optical flares. We interpret these results within the extended shock-in-jet scenario, able to explain the overall features observed here for the polarization of the blazar sample.

Accepted for publication in MNRAS

References in corpus (20)

• Probing the Inner Jet of the Quasar PKS 1510-089 with Multi-waveband Monitoring during Strong Gamma-ray Activity
• RoboPol: The optical polarization of gamma-ray--loud and gamma-ray--quiet blazars
• RoboPol: Connection between optical polarization plane rotations and gamma-ray flares in blazars
• Optical polarization of high-energy BL Lac objects
• Multifrequency variability of the blazar AO 0235+164. The WEBT campaign in 2004-2005 and long-term SED analysis
• RoboPol: Do optical polarization rotations occur in all blazars?
• RoboPol: AGN polarimetric monitoring data
• Multiwavelength behaviour of the blazar 3C279: decade-long study from $\gamma$-ray to radio
• Optical EVPA rotations in blazars: testing a stochastic variability model with RoboPol data
• Long-term optical polarization variability and multiwavelength analysis of Blazar Mrk 421
• Polarization swings in blazars
• A peculiar multi-wavelength flare in the Blazar 3C 454.3
• Spectroscopic monitoring of the BL Lac object AO 0235+164
• Infrared properties of blazars: putting the GASP-WEBT sources into context
• Short Timescale Photometric and Polarimetric Behavior of two BL Lacertae Type Objects
• Redshift determination of the BL Lac object 3C66A by the detection of its host galaxy cluster at $z=0.340$
• A statistical study of the optical spectral variability in gamma-ray blazars
• A Model of Polarisation Rotations in Blazars from Kink Instabilities in Relativistic Jets
• Polarimetric Properties of Blazars Caught by the WEBT
• Impact of seeing and host galaxy into the analysis of photo-polarimetric microvariability in blazars - Case study of the nearby blazars 1ES 1959+650 and HB89 2201+044

Cited by in corpus (4)

• Quasi-simultaneous Optical Flux and Polarization Variability of the Binary Super Massive Black Hole Blazar OJ 287 from 2015 to 2023: Detection of an Anticorrelation in Flux and Polarization Variability
• Polarization Studies in Different Blazar Types. Part I: Correlation between Optical Brightness and Polarization Degree
• Detectability of Polarized Gamma-ray Emission from Blazar Flares with COSI
• Automating the detection of polarization angle rotations in blazars. Re-analysis of RoboPol data reveals 27 new rotations