We present a polarization variability analysis of a sample of 26 γ-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 ∼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.

We present the results of a long-term periodicity search in a sample of 𝛾-ray blazars within a multiwavelength context. These blazars have been selected from the Steward Observatory sample as part of its optical monitoring program between 2008 and 2018. We study 15 sources with a temporal coverage in their optical total and polarized emission sufficiently large (> 9 years) to perform a reliable long-term periodicity analysis. We collect data from several observatories to extend the coverage, enabling the search of longer periods. In addition, data are also gathered in the high-energy (𝐸 > 100 MeV) 𝛾-ray band from the Fermi Large Area Telescope; and in the 15-GHz radio band from the Owens Valley Radio Observatory. We identify 5 promising candidates to host quasi-periodic emission, AO 0235+164, PKS 1222+216, Mrk 501, BL Lacertae and 1ES 2344+514 with periods in one or more bands and statistical significances ∼3𝜎 after trial factor correction. AO 0235+164 shows a period of ∼8.2 years in the R band; PKS 1222+216 has a quasi-periodic modulation in its total and polarized optical emission of ∼1.6 years; Mrk 501 displays a ∼5-year quasi-periodicity in optical and radio wavelengths; BL Lacertae presents a period of ∼1.8 years in its polarized emission; and 1ES 2344+514 shows a hint of a ∼5.5-year period in its optical R band. We interpret these results in the framework of the most common models and scenarios, namely the presence of a binary supermassive black hole system; or geometrical effects like helical or precessing jets.