Phase measurements as a function of time with the best-fit quadratic model with and without the XMM-Newton observation. The solid curves represent the posterior median solutions obtained using UltraNest, while the shaded regions indicate the corresponding 1σ credible intervals derived from the posterior distributions. The error on the phase measurement corresponds to total phase uncertainty, σ_tot, which includes the intrinsic phase-jitter term (σ_int) added in quadrature to the statistical uncertainty. The bottom panels show residuals with respect to the best-fit model with and without the XMM-Newton observation.