Optical polarization of high-energy BL Lacertae objects

Abstract

Context. We investigate the optical polarization properties of high-energy BL Lac objects using data from the RoboPol blazar monitoring program and the Nordic Optical Telescope. Aims. We wish to understand if there are differences in the BL Lac objects that are detected with the current-generation TeV instruments compared to those that have not yet been detected. Methods. We use a maximum likelihood method to investigate the optical polarization fraction and its variability in these sources. In order to study the polarization position angle variability, we calculate the time derivative of the electric vector position angle (EVPA) change. We also study the spread in the Stokes Q/I − U/I-plane and rotations in the polarization plane. Results. The mean polarization fraction of the TeV-detected BL Lacs is 5% while the non-TeV sources show a higher mean polarization fraction of 7%. This difference in polarization fraction disappears when the dilution by the unpolarized light of the host galaxy is accounted for. The TeV sources show somewhat lower fractional polarization variability amplitudes than the non-TeV sources. Also the fraction of sources with a smaller spread in the Q/I − U/I-plane and a clumped distribution of points away from the origin, possibly indicating a preferred polarization angle, is larger in the TeV than in the non-TeV sources. These differences between TeV and non-TeV samples seems to arise from differences between intermediate and high spectral peaking sources instead of the TeV detection. When the EVPA variations are studied, the rate of EVPA change is similar in both samples. We detect significant EVPA rotations in both TeV and non-TeV sources, showing that rotations can occur in high spectral peaking BL Lac objects when the monitoring cadence is dense enough. Our simulations show that we cannot exclude a random walk origin for these rotations. Conclusions. These results indicate that there are no intrinsic differences in the polarization properties of the TeV-detected and nonTeV-detected high-energy BL Lac objects. This suggests that the polarization properties are not directly related to the TeV-detection, but instead the TeV loudness is connected to the general flaring activity, redshift, and the synchrotron peak location.

Optical polarization properties of AGNs with significant VLBI–Gaia offsets

“…We note that the preference of EVPA to be aligned with the jet has been reported previously in a number of publications but did not result in a conclusive outcome (e.g. Rusk & Seaquist 1985;Lister & Smith 2000;Algaba et al 2011;Hovatta et al 2016;Angelakis et al 2017). The Ψ = 180 • AGN are found to have the typical level of fractional linear optical polarization several times lower than the Ψ = 0 • case (Figure 3, Figure 4, Table 1).…”

A parsec-scale wobbling jet in the high-synchrotron peaked blazar PG 1553+113

“…Although several multiwavelength (MWL) observing campaigns of PG 1553+113 (e.g. Raiteri et al 2015;Hovatta et al 2016;Itoh et al 2016;Raiteri et al 2017) were conducted in recent years, only sparse VLBI observations of PG 1553+113 can be found in the literature, including the 15 GHz observations within the MOJAVE database 2 and the TeV Blazars very long baseline array (VLBA) monitoring program led by B. G. Piner and P. G. Edwards 3 . For these reasons, we conducted a systematic multi-frequency (15, 24, and 43 GHz) VLBA monitoring of PG 1553+113 covering a period between two consecutive maxima in the γ-ray activity during 2015-2017.…”

Jet Precession Driven by a Supermassive Black Hole Binary System in the BL Lac Object PG 1553+113

“…Previous interferometric radio observations of PG 1553+113 revealed a very compact corejet structure extending to the northeast direction (e.g., Tiet, Piner & Edwards 2012;Piner & Edwards 2014). This source has been studied in several campaigns that investigated its multi-wavelength and polarimetric variabilities (e.g., Ackermann et al 2015;Raiteri et al 2015;Itoh et al 2016;Raiteri et al 2017), leading to detections of significant rotation in the optical electric vector position angles (EVPAs) that are coincident with γ-ray flares (e.g., Blinov et al 2016;Hovatta et al 2016;Jermak et al 2016).…”