Top panel – Best fit of the X-ray light curve, using a linear term and one sinusoidal modulation. The vertical dotted lines bracket the time range likely contaminated by the flare activity and excluded in the fitting, as discussed in Sect. 2.2.2. The fitted period is P = 4.2 ± 0.1 days. Bottom panel – Probability Distribution Function of the fitted periods recovered after 10 000 time shufflings. The FAP is ~2%.