Gaussian process fitting results of the QPO+DRW model for NGC 1566. Top left: Background-subtracted XMM-Newton EPIC light curve of NGC 1566 in the 0.2–10 keV band (black dots with error bars), binned at 100 s. The light curve corresponds to the obs-ID 0800840201, for which NGC 1566 was in the outburst phase. The green curves represent the long-term trend modeled by a shewed Gaussian function, which describes the outburst profile well. The yellow curve is the sum of the best-fitting QPO+DRW model and the trend. Top right: Same as the top-left panel, but with the long-term trend (i.e., the green curve in the top panel) subtracted. Bottom left: Posterior distribution of the QPO period yielding $P_{\mathrm{QPO}}=(1.{78}_{-0.14}^{+0.17})\times {10}^4\mathrm{s}$. Bottom right: Best-fitting PSDs for the QPO+DRW model (blue curve) and the noise model (purple dots). The shaded regions indicate the 1σ confidence intervals.