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

¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, RM, Italy
e-mail: tommaso.aniello@inaf.it
² “Sapienza” Università di Roma – Dip. di Fisica, P.le A. Moro 5, 00185 Roma, Italy
³ Università degli Studi di Roma “Tor Vergata” – Dip. di Fisica, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁴ INFN – Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁵ University of Maryland – Dept. of Astronomy, College Park, MD 20742, USA
⁶ NASA – Goddard Space Flight Center, Greenbelt Rd. 8800, Greenbelt, MD 20771, USA
⁷ ASI – Space Science Data Center, Via del Politecnico snc, 00133 Roma, Italy
⁸ Scuola Normale Superiore di Pisa, P.zza dei Cavalieri 7, 56126 Pisa, Italy

Received: 6 February 2024
Accepted: 5 April 2024

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 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.