\beta\right)},$$ where $p$ is the polarization fraction and $\alpha$ and $\beta$ determine the shape of the Beta distribution $B\left(\alpha, \beta\right)$. If the parameters $a, \beta$ of this distribution are known, the mean polarization fraction and the intrinsic modulation index are then given by

$$p_\text{int}=\frac{\alpha}{\alpha + \beta}$$ and $$m_\text{int}=\frac{\sqrt{Var}}{p_\text{int}}=\frac{\sqrt{\frac{\alpha\beta}{\left(\alpha + \beta\right)^2 \left(\alpha + \beta
       +1 \right)}}}{\frac{\alpha}{\alpha + \beta}},$$ where $Var$ is the variance of the distribution. Details of the method are described in Appendix A of @2016MNRAS.457.2252B. The intrinsic mean polarization fraction of S2 0109+22 is $0.090^{+0.010}_{-0.008}$, which is higher than the sample mean values of $0.054\pm0.008$ and $0.079\pm0.009$ obtained for the TeV and non-TeV BL Lac objects in . Similarly, the intrinsic modulation index of the polarization fraction $0.54^{+0.08}_{-0.06}$ is higher than the sample mean values for the TeV ($0.29\pm0.03$) and non-TeV ($0.38\pm0.04$) sources.

The polarization angle variability can be quantified by calculating the derivative of the polarization angle variations. First we account for the n$\pi$ ambiguity of the polarization angle by requiring that each subsequent point is within 90$^\circ$ from the previous observation. We obtain a median derivative of 2.4 degrees per day, which translates to 3.3 degrees per day in the source frame when multiplied by $(1+z)$ ($z=0.35$, 