Analysis results obtained with the Xronos software. Left panel: PG 1553+113 PSD of X-ray LC obtained with the powerspec package of Xronos. Middle panel: best-fit period (solid line) of PG 1553+113 X-ray LC obtained with the efsearch task, along with its numerical value in seconds corresponding to ∼1.4 years (see text). Right panel: epoch folding of PG 1553+113 X-ray LC obtained with the efold method on the basis of the best-fit period determined by efsearch.