Stochastic model of optical variability of BL Lacertae

TL;DR

This study models the optical variability of BL Lacertae over 22 years using a stochastic approach that combines a steady polarized source with multiple variable components, fitting observational data through Monte Carlo simulations.

Contribution

It introduces a stochastic model with a steady polarized source and variable components, fitted to long-term optical data of BL Lacertae, providing insights into its polarization variability.

Findings

The steady source has ~40% polarization aligned with the jet.

Approximately 10 variable sources contribute to polarization variability.

The model successfully reproduces observed photometric and polarimetric variability.

Abstract

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 Hagen-Thorn et al. earlier. 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 chi-square 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.