Quiescent level variability. In the upper panels, we show the 0.2-1 keV light curves from all XMM-Newton observations with QPEs, together with their best-fitting models comprising a constant and a series of Gaussian functions to describe the quiescent level and QPEs respectively. The lower panels show the corresponding residual light curves re-binned by a factor of 8, together with two models, a constant (dashed lines) and a sine function (solid lines) with a period equal to the average observation-dependent recurrence time.