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We have studied long-term optical, ultraviolet (UV) and X-ray observations ofOJ 287 collected with the UVOT and XRT instruments mounted on board the Swiftsatellite to quantify spectral and temporal variability patterns observedduring different activity states. We characterized the flux variations usingthe data collected during almost 11 yr of the monitoring of the blazar.Significant variability of the blazar has been detected both in the flux andspectral index from the optical to X-ray regimes. We noted that the variabilitypatterns observed in the optical range are more pronounced than the ones in theX-ray band. There is no clear relation between the optical/UV and X-rayemission, neither during the quiescence state nor during outbursts. The mostsignificant flares in the optical/UV regime were detected in 2015 December-2016January. The shortest variability time-scale is one day and it is limited bythe observation pointing. A low activity state of OJ 287 was observed at theend of 2014, while the beginning of 2015 revealed a flat X-ray spectrum, whichhas been observed for the first time. On one hand, this can be a spectralupturn where the synchrotron and inverse Compton components meet, but on theother hand, it can be generated by an additional emission component. Thespectral studies have not revealed any bluer-when-brighter orredder-when-brighter chromatism in the colour-magnitude diagram for OJ 287 inany state of the source's activity. A harder-when-brighter behaviour wasnoticed for OJ 287 only in the case of the X-ray observations.

Characterizing long-term optical, ultraviolet and X-ray variability in different activity states of OJ 287