Polarization Studies in Different Blazar Types. I. Correlation Between Optical Brightness and Polarization Degree

Abstract We present an analysis of the cross-correlation between optical brightness and polarization degree in different types of blazars. The aim is to identify objects with simultaneous and consistent changes in characteristics and to determine if this behavior relates to the types of objects studied. The analysis includes 23 objects: 11 Flat Spectrum Radio Quasars (FSRQs), 11 BL Lacs, and 1 radio galaxy. Dense overlapping observation series in the R band were used, collected over more than 10 yr as part of a monitoring program for bright blazars at St. Petersburg State University. The cross-correlation analysis procedure is detailed, including a method for assessing significance based on Monte Carlo simulations of synthetic light curves modeled using a Damped Random Walk. Significant correlations were found for 5 FSRQs and 1 BL Lac. No significant correlation was detected for 10 BL Lacs and 6 FSRQs. One object did not yield a reliable estimate. Based on the current results, we cannot claim that the observed difference in the behavior of these emission characteristics for different classes of blazars is significant. It is possible that observed correlations may be explained by the contribution of simultaneous flare events to the changes in flux and polarization degree curves, which occur more frequently in FSRQs.