Promise of persistent multi-messenger astronomy with the blazar OJ 287
  Successful observations of the seven predicted bremsstrahlung flares from the
unique bright blazar OJ 287 firmly point to the presence of a nanohertz
gravitational wave (GW) emitting supermassive black hole (SMBH) binary central
engine. We present arguments for the continued monitoring of the source in
several electromagnetic windows to firmly establish various details of the SMBH
binary central engine description for OJ 287. In this article, we explore what
more can be known about this system, particularly with regard to accretion and
outflows from its two accretion disks. We mainly concentrate on the expected
impact of the secondary black hole on the disk of the primary on December 3,
2021, and the resulting electromagnetic signals in the following years. We also
predict the times of exceptional fades and outline their usefulness in the
study of the host galaxy. A spectral survey has been carried out, and spectral
lines from the secondary were searched for but not found. The jet of the
secondary has been studied and proposals to discover it in future VLBI
observations are mentioned. In conclusion, the binary black hole model explains
a large number of observations of different kinds in OJ 287. Carefully timed
future observations will be able to provide further details of its central
engine. Such multi-wavelength and multidisciplinary efforts will be required to
pursue multi-messenger nanohertz GW astronomy with OJ 287 in the coming
decades.
