A Multi-wavelength View of OJ 287 Activity in 2015-2017: Implications of
  Spectral Changes on Central-engine Models and MeV-GeV Emission Mechanism
  A diverse range of peculiar properties across the domains of observation have
made OJ 287 one of the best-explored BL Lac objects on the issues of
relativistic jets and accretion physics as well as the strong theory of
gravity. We here present a brief compilation of observational results from the
literature and inferences/insights from the extensive studies but focus on the
interpretation of its $\sim$ 12-yr QPOOs and high energy emission mechanisms.
The QPOOs in one model are attributed to the disk-impact related to dynamics of
the binary SMBHs while alternative models attribute it to the geometrical
effect related to the precession of a single jet or double jets. We discuss
implications of the new spectral features reported during the 2015--2017
multi-wavelength high activity of the source -- a break in the NIR-optical
spectrum and hardening of the MeV-GeV emission accompanied by a shift in the
location of its peak, in the context of the two. The reported NIR-optical break
nicely fits the description of a standard accretion disk emission from an SMBH
of mass $\sim~10^{10}~M_\odot$ while the time of its first appearance in
end-May 2013 (MJD 56439) is in close coincidence with the time of impact
predicted by the disk-impact binary SMBH model. This spectral and temporal
coincidence with the model parameters of the disk-impact binary SMBH model
provides independent evidence in favor of the model over the geometrical models
which argue a total central-engine mass in the range of $\rm 10^{7-9}~M_\odot$.
On the other hand, the MeV-GeV spectral change is naturally reproduced by the
inverse Compton scattering of photons from the broad-line region and is
consistent with the detection of broad emission lines during the previous
cycles of quasi-periodic outbursts. Combining this with previous SED studies
suggests that in OJ 287, MeV-GeV emission results from external Comptonization.
