Kapanadze, B.B.KapanadzeVERCELLONE, STEFANOSTEFANOVERCELLONEROMANO, PatriziaPatriziaROMANOHughes, P.P.HughesAller, M.M.AllerAller, H.H.AllerKapanadze, S.S.KapanadzeTabagari, L.L.Tabagari2020-11-172020-11-1720180035-8711http://hdl.handle.net/20.500.12386/28400We present the results of a detailed X-ray timing and spectral analysis of the BL Lacertae source OJ 287 with X-ray telescope (XRT) onboard Neil Gehrels Swift Observatory, focused on the period of its significantly enhanced X-ray flaring activity during 2016 October-2017 April. In this epoch, the 0.3-10 keV count rate from the XRT observations showed an increase by a factor of ∼10 compared to the quiescent level observed in 2016 April-May, and the mean X-ray flux was a factor of 4.5 higher than in previous years. The source underwent high X-ray flaring activity on weekly time-scales and showed 32 instances of 0.3-10 keV intraday variability (detected within the exposures shorter than 1 ks in the majority of cases) with fractional variability amplitudes of 7-60 per cent. Most of the 0.3-10 keV spectra spectra fitted well with a simple power law, yielding a wide range of the 0.3-10 keV photon index Γ = 1.90-2.90. We found 29 spectra showing an upward curvature due to the significant contribution made by the X-ray photons of inverse Compton origin. The spectral variability of OJ 287 was characterized by the dominance of a `softer-when-brighter' spectral trend, explained by the emergence of a new soft component during X-ray flares. Similar to X-rays, the source underwent a strong outburst by factors of 4.6-6.5 in the optical-ultraviolet energy range which showed a positive correlation with the X-ray emission, indicating its origin to be related to the same electron population, predominantly via the synchrotron mechanism.STAMPAenArticle10.1093/mnras/sty18032-s2.0-85052556847000442567900033https://academic.oup.com/mnras/article/480/1/407/50500742018MNRAS.480..407KFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_6 Stars and stellar systems