Changepoint analysis of a set of polarized intensity components. (A) The intensities of four polarized components are replotted from Fig. 1 A. Dashed vertical thin lines indicate individual finally identified intensity transitions, t₁–t₂₂, and a false positive one between t₄–t₅. (B) The ratio between LLR and the 95% confidence threshold is plotted for each identified transition. The equation to calculate LLR is given at the bottom (Lewis and Lu, 2019a), in which m_r is the number of photons detected during the duration τ from the beginning of a time segment and the evaluated time point, whereas N_r is the total number of photons detected during the total duration T of the segment. The final LLR is the sum of those for the individual four polarized components of intensity. As expected, all values are above one. By definition, for a 95% confidence detection threshold, 1 false positive occurs for every 20 identified transitions on average. Here, among the total 23 positives, the one pointed at by the orange arrow is judged to be false on the basis that it is a transition within C₁ itself (see Results). Consequently, the number of finally identified intensity transitions is reduced by one to 22. LLR: log maximum likelihood ratio.