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

TL;DR

This study analyzes over 11 years of optical, UV, and X-ray data of OJ 287 to understand its variability patterns across different activity states, revealing complex flux and spectral changes without clear emission correlations.

Contribution

It provides a comprehensive long-term multi-wavelength variability analysis of OJ 287, highlighting new spectral features and variability timescales during different activity states.

Findings

Significant flux and spectral variability detected across optical to X-ray bands.

No clear correlation between optical/UV and X-ray emissions during different states.

First observation of a flat X-ray spectrum during a low activity state.

Abstract

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