Testing X-ray periodicity and the long-term trend in PG 1553+113 via targeted Swift-XRT monitoring

¹ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634-0978, USA
² Department of Astronomy, University of Virginia, P.O. Box 400325 Charlottesville, VA 22904, USA
³ Deutsches Elektronen-Synchrotron DESY, Platanenallee 6, 15738 Zeuthen, Germany
⁴ IPARCOS and Department of EMFTEL, Universidad Complutense de Madrid, E-28040, Madrid, Spain
⁵ Julius-Maximilians-Universität Würzburg, Fakultät für Physik und Astronomie, Emil-Fischer-Str. 31, D-97074 Würzburg, Germany

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 March 2026
Accepted: 7 April 2026

Abstract

Context. PG 1553+113 is a blazar with the most significantly detected periodic pattern in its multiwavelength (MWL) emission, making it one of the most promising candidates for hosting a supermassive black hole binary.

Aims. The presence of this periodic behavior in the X-ray band remains under debate, largely due to the lack of continuous monitoring. This has led to differing conclusions across previous studies. We examined whether the recently identified linear long-term trends in the γ-ray and optical bands also exist in the X-ray regime.

Methods. We evaluated the ∼2.1-year period in the X-ray light curve of PG 1553+113 using two dedicated monitoring campaigns with Swift-XRT and UVOT, guided by predictions of future oscillation phases. We also examined whether the long-term trend is present in X-rays, the potential periodic behavior of the X-ray power-law photon index, and its potential correlation with the X-ray flux.

Results. We find tentative evidence for a correlation between the predicted high-emission states in the γ-ray band and those observed in the X-ray and UV bands. However, we do not find strong evidence of the same periodic pattern in the X-ray band. In addition, we find that the X-ray light curve is consistent with the presence of a long-term linear trend, in agreement with those previously reported in the γ-ray, optical, and radio bands.

Conclusions. Overall, these results indicate that the X-ray emission likely shares the same long-term behavior observed in the γ-ray and optical bands. Nevertheless, the pronounced stochastic variability that characterizes the X-ray light curve limits our ability to draw firm conclusions regarding the presence of this periodic behavior.