Transitional spectral behaviour in Blazar OJ 287: A comprehensive multi-epoch SED study

ABSTRACT We present a comprehensive multiwavelength investigation of BL Lac object OJ 287 using Swift and Fermi observatories spanning 2008–2025. The source exhibits significant flux variability across optical-UV, X-ray, and $\gamma$-ray regimes, with outbursts observed in optical, UV, and X-ray wavelengths. Variability and correlation analyses of the long-term light curves reveal strong correlations among optical, UV, and X-ray bands, whereas there is no significant correlation between $\gamma$-ray and other bands. Using a decade-long data set (MJD 57382–60448), we analysed X-ray spectral variability across different flux states (flare, intermediate, and quiescent). Multiwavelength spectral energy distributions (SEDs) were constructed using a one-zone leptonic model to investigate the transitional nature of the X-ray spectral position in the broad-band. The analysis reveals OJ 287’s consistent flux-dependent transitional spectral behaviour: quiescent states show hard X-ray spectra dominated by inverse Compton emission; intermediate states exhibit contributions from both inverse Compton and synchrotron components with moderately hard spectra; flaring events are characterized by predominantly synchrotron emission, resulting in soft X-ray spectra. Broad-band SED modelling captures the systematic evolution of model parameters across different activity states, with correlation analysis revealing strong positive correlations between synchrotron peak frequency, synchrotron-self Compton peak frequency, and X-ray flux, providing quantitative evidence for the changing dominance of emission components within the blazar’s relativistic jet.