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

Abstract

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 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 gamma -ray, UV, and optical bands. We aim to investigate the variability pattern of the 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 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 sim 1.4 years, which is about 35<!PCT!> shorter than the one observed in the gamma -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.