Rms-normalized PDS of SXP31.0 during the QPO-active interval for NuObs1 (3–79 keV) computed from the first 32.8 ks of the observation where the QPO is most prominent. The best-fit model is shown in red, with the green dashed line for the QPO component and the blue dashed line for the shoulder component. Panel (b) shows residuals to the model with only the QPO and spin frequency, while panel (c) shows residuals after adding the shoulder component.