2025MNRAS.541.3008Z

OJ 287 is one of the best-observed blazars in the X-ray regime. In order to investigate the variability characteristics, we have compiled the long-term X-ray data monitored by Swift-X-Ray Telescope, and conducted a thorough analysis to explore both the quasi-periodic oscillation (QPO) and the spectral variability within these data. Our findings reveal, for the first time, the presence of a strong 39-d QPO signal with a $5.4 \sigma$ significance level during the outburst stage. The QPO endures for approximately four cycles, encompassing a total of four peaks. We have conducted a further analysis of the spectral evolution in the X-ray band through the relationship between hardness ratio and count rate. OJ 287 exhibits a typical feature, i.e. as the source becomes brighter, the spectrum progressively softens, but the spectrum changes increasingly slowly until it tends to be stable, and the entire process of change is non-linear. We have termed this characteristic spectral behaviour the 'softer-stable-when-brighter' (SSWB) phenomenon and quantitatively described it using a newly developed non-linear function, which captures its essential properties. To account for the observed X-ray spectral behaviour, we have proposed a two-component emission model featuring quasi-constant spectral indices. The X-ray emission is predominantly composed of synchrotron and inverse Compton components, with the synchrotron emission playing a dominant role in the variability of the X-ray band, especially during outbursts. The model effectively characterizes the spectral features associated with the SSWB phenomenon, providing both qualitative and quantitative descriptions.