Multiwavelength temporal and spectral variability of the blazar OJ 287 during and after the 2015 December flare: a major accretion disc contribution

Abstract

We present a multi-wavelength spectral and temporal analysis of the blazar OJ\n287 during its recent activity between December 2015 -- May 2016, showing\nstrong variability in the near-infrared (NIR) to X-ray energies with detection\nat $\\gamma$-ray energies as well. Most of the optical flux variations exhibit\nstrong changes in polarization angle and degree. All the inter-band time lags\nare consistent with simultaneous emissions. Interestingly, on days with\nexcellent data coverage in the NIR--UV bands, the spectral energy distributions\n(SEDs) show signatures of bumps in the visible--UV bands, never seen before in\nthis source. The optical bump can be explained as accretion-disk emission\nassociated with the primary black hole of mass $\\sim \\rm 1.8 \\times10^{10}\nM_{\\odot}$ while the little bump feature in the optical-UV appears consistent\nwith line emission. Further, the broadband SEDs extracted during the first\nflare and during a quiescent period during this span show very different\n$\\gamma$-ray spectra compared to previously observed flare or quiescent\nspectra. The probable thermal bump in the visible seems to have been clearly\npresent since May 2013, as found by examining all available NIR-optical\nobservations, and favors the binary super-massive black hole model. The\nsimultaneous multi-wavelength variability and relatively weak $\\gamma$-ray\nemission that shows a shift in the SED peak is consistent with $\\gamma$-ray\nemission originating from inverse Compton scattering of photons from the line\nemission that apparently contributes to the little blue bump.\n