Null-hypothesis probability of the χ² test for time variability of the 𝒬 (red) and 𝒰 (blue) Stokes parameters with the constant model for different numbers of time bins for Obs. 4. The left and right vertical axes correspond to the probability values in logarithmic and linear scales, respectively. The green shaded area indicates that the null-hypothesis probability is above the 1% significance level. The dashed and dotted black lines located in the middle of the panel represent 1% and 3σ (99.73%) probability, respectively.