Results of the simulations carried out to investigate the significance of the period at  ~120 days found from the Swift/BAT data. The upper panel shows the value of the period for each simulation (the number of the simulation is reported on the x-axis). The middle panel reports the probability that the detection of a period is false in each simulation (i.e., a value of 0.1 corresponds to a detection significance of 90% c.l.), and the lower panel the power of each peak in the corresponding LS diagram.