2009A&A...503..103B

We use optical photometric and polarimetric data of BL Lacertae that cover a period of 22 years to study the variability of the source. The long-term observations are employed for establishing parameters of a stochastic model consisting of the radiation from a steady polarized source and a number of variable components with different polarization parameters, proposed by the authors early. We infer parameters of the model from the observations using numerical simulations based on a Monte Carlo method, with values of each model parameter selected from a Gaussian distribution. We determine the best set of model parameters by comparing model distributions to the observational ones using the χ² criterion. We show that the observed photometric and polarimetric variability can be explained within a model with a steady source of high polarization, ∼40%, and with direction of polarization parallel to the parsec scale jet, along with 10±5 sources of variable polarization.