a: Swift/UVOT light curve in the U-band. b: Swift/XRT lightcurve. The colored lines show the four different assumed flux evolutions, which are used as an input for the pulse period evolution: constant flux (orange), linear brightening and dimming trend (red), measured XRT lightcurve (green), and extrapolated X-ray super-orbital period profile (blue). The dotted line represents the estimated count rate at the Eddington limit. c: pulse period evolution as measured by XMM-Newton (circles) and NuSTAR (diamonds). Superimposed are the best-fit models for the four different input lightcurves in the same colors as in panel b. The brown dotted-dashed lines indicate the times of observations when no pulsations where seen. d: residuals with respect to the linear brightening and dimming input, e: residuals with respect to a constant input, f: residuals with respect to the original lightcurve as input, and g residuals with respect to the X-ray profile input. h: measured (black and gray) and predicted (orange) pulse period derivative Ṗ. The model is based on the constant flux model. i: pulsed fraction in the 3−10 keV energy band. Upper limits are denoted by downward pointing arrows. For details see text.