Unveiling the periodic variability patterns of the X-ray emission from the blazar PG 1553+113

The search for periodicity in the multiwavelength, highly variable emission of blazars is a key feature to understanding dynamical processes at work in this class of active galactic nuclei. The blazar PG 1553+113 is an attractive target due to the evidence of periodic oscillations observed at different wavelengths, with a solid proof of a 2.2-year modulation detected in the γ-ray, UV, and optical bands. We aim to investigate the variability pattern of the PG 1553+113 X-ray emission using a more than 10-year-long light curve in order to robustly assess the presence or lack of a periodic behavior, evidence of which is only marginal so far. We conducted detailed statistical analyses, studying in particular the variability properties of the X-ray emission of PG 1553+113 by computing the Lomb-Scargle periodograms, which are suited for the analyses of unevenly sampled time series, and adopting epoch-folding techniques. We find a modulation pattern in the X-ray light curve of PG 1553+113 with a period of ∼1.4 years, which is about 35% shorter than the one observed in the γ-ray domain. Our finding is in agreement with the recent spectro-polarimetric analyses and supports the presence of more dynamical phenomena simultaneously at work in the central engine of this quasar.