Supermassive binary black holes and the case of OJ 287
  Supermassive binary black holes (SMBBHs) are laboratories par excellence for
relativistic effects, including precession effects in the Kerr metric and the
emission of gravitational waves. Binaries form in the course of galaxy mergers,
and are a key component in our understanding of galaxy evolution. Dedicated
searches for SMBBHs in all stages of their evolution are therefore ongoing and
many systems have been discovered in recent years. Here we provide a review of
the status of observations with a focus on the multiwavelength detection
methods and the underlying physics. Finally, we highlight our ongoing,
dedicated multiwavelength program MOMO (for Multiwavelength Observations and
Modelling of OJ 287). OJ 287 is one of the best candidates to date for hosting
a sub-parsec SMBBH. The MOMO program carries out a dense monitoring at >13
frequencies from radio to X-rays and especially with Swift since 2015. Results
so far included: (1) The detection of two major UV-X-ray outbursts with Swift
in 2016/17 and 2020; exhibiting softer-when-brighter behaviour. The non-thermal
nature of the outbursts was clearly established and shown to be synchrotron
radiation. (2) Swift multi-band dense coverage and XMM-Newton spectroscopy
during EHT campaigns caught OJ 287 at an intermediate flux level with
synchrotron and IC spectral components. (3) Discovery of a remarkable, giant
soft X-ray excess with XMM and NuSTAR during the 2020 outburst. (4) Spectral
evidence (at 2sigma) for a relativistically shifted iron absorption line in
2020. (5) The non-thermal 2020 outburst is consistent with an after-flare
predicted by the SMBBH model of OJ 287.
