Coordinated multisite observations of the variability of BL Lac

Abstract

The authors present the results of multisite observations of the optical polarization of BL Lac, together with some flux measurements, at a time when BL Lac was relatively faint. The level of polarization varies in a manner similar to that reported by Moore et al., although the position angle behaviour is quite different, varying only a small amount from a mean of 28°. Models are presented to account for the observed polarization behaviour of BL Lac. The best model is one consisting of two polarized components, one having a variable polarization, intensity and random position angle, and the other a fixed polarization, intensity and position angle. The authors tentatively identify this latter component as being associated with the inner region of the optical jet of BL Lac.