Top panel: pulse phase delays as a function of time computed by epoch-folding ~250 s long intervals of NuSTAR data, together with the best-fit model (red dotted line, see text). Middle panel: residuals in ms with respect to the best-fit orbital solution including a linear model for the pulse phase delays. Bottom panel: residuals in ms with respect to the best-fit orbital solution including a quadratic model for the pulse phase delays.