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

TL;DR

This study analyzes multi-wavelength variability of blazar OJ 287 during and after the 2015 flare, revealing accretion disk signatures and spectral features supporting a binary supermassive black hole model.

Contribution

It provides the first detection of optical-UV bumps in OJ 287's SEDs, linking them to accretion disk emission and line features, supporting the binary black hole hypothesis.

Findings

Optical-UV bumps indicate accretion disk and line emission.

Spectral energy distributions vary between flare and quiescent states.

Thermal bump has been present since May 2013, supporting binary SMBH model.

Abstract

We present a multi-wavelength spectral and temporal analysis of the blazar OJ 287 during its recent activity between December 2015 -- May 2016, showing strong variability in the near-infrared (NIR) to X-ray energies with detection at $\gamma$-ray energies as well. Most of the optical flux variations exhibit strong changes in polarization angle and degree. All the inter-band time lags are consistent with simultaneous emissions. Interestingly, on days with excellent data coverage in the NIR--UV bands, the spectral energy distributions (SEDs) show signatures of bumps in the visible--UV bands, never seen before in this source. The optical bump can be explained as accretion-disk emission associated with the primary black hole of mass $\sim \rm 1.8 \times10^{10} M_{\odot}$ while the little bump feature in the optical-UV appears consistent with line emission. Further, the broadband SEDs extracted during the first flare and during a quiescent period during this span show very different $\gamma$-ray spectra compared to previously observed flare or quiescent spectra. The probable thermal bump in the visible seems to have been clearly present since May 2013, as found by examining all available NIR-optical observations, and favors the binary super-massive black hole model. The simultaneous multi-wavelength variability and relatively weak $\gamma$-ray emission that shows a shift in the SED peak is consistent with $\gamma$-ray emission originating from inverse Compton scattering of photons from the line emission that apparently contributes to the little blue bump.