#  Supermassive binary black holes and the case of OJ 287

###### Abstract

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 $>$13 frequencies from radio to X-rays and especially with Swift
since 2015. Results so far included: (1) The detection of two major UV-X-ray
outbursts with Swift in 2016/17, and 2020; exhibiting softer-when-brighter
behaviour. The non-thermal nature of the outbursts was clearly established and
shown to be synchrotron radiation. (2) Swift multi-band dense coverage and
XMM-Newton spectroscopy during EHT campaigns caught OJ 287 at an intermediate
flux level with synchrotron and IC spectral components. (3) Discovery of a
remarkable, giant soft X-ray excess with XMM and NuSTAR during the 2020
outburst. (4) Spectral evidence (at 2$\sigma$) 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.

S. KOMOSSA1, S. CIPRINI2,3, L. DEY4, L.C. GALLO5, J.L. GÓMEZ6, A. GONZALEZ5,

D. GRUPE7, A. KRAUS1, S.J. LAINE8, M.L. PARKER9,10, M.J. VALTONEN11,12,

S. CHANDRA13, A. GOPAKUMAR4, D. HAGGARD14,15, and M.A. NOWAK16

1Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn,
Germany
2Istituto Nazionale di Fisica Nucleare (INFN) Sezione di Roma Tor Vergata, Via
della Ricerca Scientifica 1, 00133, Roma, Italy
3 ASI Space Science Data Center (SSDC), Via del Politecnico, 00133, Roma,
Italy
4 Department of Astronomy and Astrophysics, Tata Institute of Fundamental
Research, Mumbai 400005, India
5 Department of Astronomy and Physics, Saint Mary’s University, 923 Robie
Street, Halifax, NS, B3H 3C3, Canada
6 Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n,
E-18008 Granada, Spain
7Department of Physics, Earth Science, and Space System Engineering, Morehead
State University, 235 Martindale Dr, Morehead, KY 40351, USA
8 IPAC, Mail Code 314-6, Caltech, 1200 E. California Blvd., Pasadena, CA
91125, USA
9 European Space Agency (ESA), European Space Astronomy Centre (ESAC), E-28691
Villanueva de la Canada, Madrid, Spain
10 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge
CB3 0HA, UK
11 Finnish Centre for Astronomy with ESO, University of Turku, FI-20014,
Turku, Finland
12 Department of Physics and Astronomy, University of Turku, FI-20014, Turku,
Finland
13 Centre for Space Research, North-West University, Potchefstroom 2520, South
Africa
14 Department of Physics, McGill University, 3600 rue University, Montréal, QC
H3A 2T8, Canada
15 McGill Space Institute, McGill University, 3550 rue University, Montréal,
QC H3A 2A7, Canada
16 Department of Physics, Washington University in St. Louis, One Brookings
Dr., St. Louis, MO 63130-4899, USA

##  1 INTRODUCTION

SMBBHs form in galaxy mergers which happen frequently throughout the history
of the universe (Volonteri et al. 2003). Coalescing binaries are the loudest
sources of low-frequency gravitational waves (GWs; Centrella et al. 2010,
Kelley et al. 2019). An intense electromagnetic search for wide and close
binaries in all stages of their evolution is therefore ongoing. Wide pairs can
be directly identified by spatially-resolved imaging spectroscopy. However,
indirect methods are required for detecting the most compact, most evolved
systems. These latter systems are well beyond the “final parsec” in their
evolution (Begelman et al. 1980; Colpi 2014; our Fig. 1) and they are in a
regime where GW emission contributes to, or even dominates, the shrinkage of
their orbits.

Detecting and modelling SMBBHs in all stages of their evolution from wide to
close systems allows us to address questions which are central to our
understanding of the assembly history and demography of supermassive black
holes (SMBHs), and of galaxy-SMBH formation and (co-)evolution across cosmic
times (e.g., Komossa et al. 2016). These questions are:

  * •

When does accretion start during a galaxy merger?

  * •

How long does accretion last, how much feedback is triggered, and therefore
how fast and how much do the SMBHs grow?

  * •

How much do the SMBHs’ spins change during accretion and merger?

  * •

How often are both SMBHs active?

  * •

How much accretion happens before and how much after the coalescence?

  * •

How efficient is the loss of angular momentum due to the binary’s interactions
with gas and stars (final parsec problem; Fig. 1), and how fast do the two
SMBHs coalesce?

  * •

How frequent are recoiling SMBHs and therefore, how frequent are galaxies
without central SMBHs?

  * •

What is the distribution of recoil velocities and amplitudes, and how long do
the SMBHs remain active after the kick?

Further, SMBBH searches will not only inform the future space-based GW
missions and the pulsar-timing arrays (PTAs) on the expected coalescence
rates, but will also reveal the (pre-coalescence) initial conditions in
systems that will later become major GW events.

Whenever both merging galaxies harbor an SMBH, the formation of a binary is
inevitable. The merger evolves in several stages (Begelman et al. 1980; our
Fig. 1). The early stages of galaxy merging are driven by dynamical friction.
At close separations, on the order of parsecs, the two SMBHs form a bound
pair. The further shrinkage of their orbit then depends on the efficiency of
interactions with stars and gas. Without any such interactions which carry
away energy and angular momentum, the binary would stall and may then never
coalesce within a Hubble time. This problem is known as the “final parsec
problem”. Recent simulations have shown that interactions with gas (like
molecular clouds in the center; “massive perturbers”) or with stars in
asymmetric nuclear potentials and on elongated orbits, efficiently drive the
binary beyond the final parsecs (e.g., review by Colpi 2014). At separations
well below a parsec, emission of GWs then becomes the dominant effect that
leads to efficient further orbital shrinkage, followed by the final
coalescence. This GW-driven regime can be thought of as proceeding in several
stages: the inspiral phase, the dynamical merger, and the final ringdown.
During each stage characteristic GW radiation is emitted (Centrella et al.
2010).

Figure 1: Sketch of the evolutionary stages of SMBBHs in galaxy mergers,
following Begelman et al. (1980). After (1) the merging of the galaxies due to
dynamical friction, the two SMBHs will (2) form a bound pair at separations of
the order of parsecs. As the binary hardens, (3) its orbit will shrink due to
the emission of GW radiation, leading to the final coalescence accompanied by
a strong burst of GW emission, and followed by (4) the recoil of the newly
formed single black hole. The kick velocity depends on the orbital
configuration and black hole mass ratio (therefore no particular timescale or
distance should be associated with it in the sketch).

Observing pairs and binaries of SMBHs in all stages of galaxy merger evolution
is of great interest. Given the limited space of this review, and the large
number of important observational and theoretical results which have emerged
on this topic in recent years, it is impossible to give credit to all of
these. We would therefore like to apologize in advance. We will focus on
reviewing the major observational signatures which have been used to search
for wide and close systems of SMBBHs, and we will mention some of the first-
identified and best-studied representative systems.

##  2 Wide pairs, spatially resolved

During gas-rich galaxy mergers, large amounts of gas are driven to the center
and are available for accretion onto one or both SMBHs (Mayer 2013). In the
early evolutionary stages of a galaxy merger, the two SMBHs can still be
spatially resolved (in X-rays, Chandra achieves 0.5${}^{{}^{\prime\prime}}$
resolution, which is similar to ground-based non-AO-assisted imaging
spectroscopy in the optical) and they can therefore be uniquely identified.
Accreting SMBHs reveal their presence by a number of characteristic emission
signatures. These include luminous X-ray emission from the accretion disk
itself, bright and extended jets in radio(-loud) systems, and optical emission
lines from the broad-line region (BLR) and narrow-line region (NLR) (these two
systems of gas clouds reprocess the incident continuum emission from the
accretion disk into emission lines; Peterson 1997).

In gas-rich mergers which have their cores heavily obscured by gas and dust,
X-rays are the most powerful probe of active SMBHs, since hard X-rays can
penetrate even high column densities ($N_{\rm H}$) of gas. Matter only becomes
Compton-thick at $N_{\rm H}\approx 10^{24}$ cm-2. Breakthroughs in the
observations of galaxy pairs and mergers were made by the Chandra X-ray
observatory. It was launched in 1999 and for the first time in the history of
X-ray astronomy provided us with high-resolution sub-arcsecond imaging and
spectroscopy.

About 15% of all active galactic nuclei (AGN) are radio-loud and drive
powerful radio jets that are launched in the immediate vicinity of the SMBH.
While jets often show a knotty structure, the true radio cores can be
identified by their compactness, variability and especially their flat radio
spectra. Binaries therefore reveal themselves by the presence of two radio
cores and/or two separate jet systems.

In less heavily obscured galaxy mergers, a small fraction of photons from the
accretion disk still reach the NLR, at distances of $\sim$10–1000 pc from the
core. SMBH pairs therefore can reveal their presence by two NLRs in form of
double-peaked emission lines and especially double-peaked [OIII]$\lambda$5007
that often is the brightest optical NLR line. Great progress in this field has
been made since large spectroscopic sky surveys became available. Especially,
the exceptional Sloan Digital Sky Survey (SDSS) has enabled the selection of
large numbers of [OIII] double-peakers. At the same time, it has to be kept in
mind that mechanisms other than SMBH pairs can also produce double-peaked
emission lines in single SMBH systems. These mechanisms include two-sided
outflows, two-sided jet-NLR interactions, warped galactic disks, or a single
active SMBH illuminating the insterstellar media of two galaxies. Therefore,
multiwavelength follow-up observations have been employed to confirm or reject
the binary nature of [OIII] double-peakers (e.g., Fu et al. 2011, Liu et al.
2018, Comerford et al. 2018, Rubinur et al. 2019). Only a small fraction (a
few percent) turned out to be active SMBH pairs.

Examples of wide systems of binary SMBHs in advanced galaxy mergers include
NGC 6240 identified in X-rays with Chandra imaging spectroscopy (Komossa et
al. 2003; our Fig. 2), 0402+379 (4C +37.11) identified in the radio regime
with the VLBI technique (Rodriguez et al. 2006), and SDSSJ 1502+1115 in the
optical with double-peaked [OIII] emission and multiwavelength confirmation
(Fu et al. 2011).

Figure 2: Pair of active SMBHs (blue) at the core of the galaxy NGC 6240
detected with Chandra imaging spectroscopy (image credit: NASA/CXC/Komossa et
al. 2003). The two accreting SMBHs (separated by 1 kpc) are identified by
their luminous, hard, point-like X-ray emission and their spectra. The Chandra
ACIS-S X-ray spectra of both nuclei (right panel: Southern nucleus) show the
signatures of heavily obscured but intrinsically luminous AGN including the
characteristic iron line near 6.4 keV.

##  3 Compact binaries, spatially unresolved

Nearly all the sub-parsec systems are spatially unresolved, and we rely on
indirect methods to identify them. The most common search methods are all
based on signs of semi-periodicity and are discussed in the following
sections.

name | (AGN) type | redshift | waveband | method
---|---|---|---|---
NGC 6240 | ULIRG | 0.024 | X-ray | imaging spectroscopy
0402+379 | radio galaxy | 0.055 | radio | imaging spectroscopy
SDSS J1502+1115 | Seyfert | 0.39 | optical | [OIII] double-peaks & radio imaging
OJ 287 | BL Lac | 0.306 | optical | semi-periodic light curve
Mrk 501 | BL Lac | 0.034 | radio | semi-periodic jet structure
3C 66B | radio galaxy | 0.021 | radio | semi-periodic astrometric position
PG 1302–102 | FSRQ | 0.3 | optical | semi-periodic light curve
SDSS J1201+3003 | quiescent | 0.146 | X-ray | TDE lightcurve
NGC 4151 | Seyfert | 0.003 | optical | semi-periodic light curve & broad line
Table 1: Summary of the systems mentioned in this review (upper panel:
spatially resolved SMBH pairs, lower panel: spatially unresolved SMBBH
candidates). All of them stand out in being among the first identified and
best-studied systems of their kind. Column 2 provides the classification of
the host galaxy or AGN type (ULIRG stands for ultraluminous infrared galaxy;
FSRQ for flat-spectrum radio quasar). In column 4, the waveband in which the
system was first identified is reported. Column 5 gives the method of binary
identification.

###  3.1 Semi-periodic jet structures

A number of AGN radio jets show semi-periodic deviations from a straight line.
One way to explain these observations is involving the presence of a binary
SMBH that causes either a modulation due to orbital motion of the jet-emitting
SMBH around the primary SMBH, or jet precession. This method was among the
first explored in the search for SMBBHs, and was motivated by the structures,
bendings, and helicities observed in radio jets (Begelman et al. 1980)111See
Saslaw et al. (1974) for a discussion of SMBBH formation in three-body-
interactions and radio constraints at that time.. Radio interferometry has
provided us with the highest-resolution observations of jets over decades. Mrk
501 is an early example of an AGN with helical radio jet structure,
interpreted as Kelvin-Helmholtz instability driven at the origin through the
orbital motion of an SMBBH (Conway & Wrobel 1995). Hydrodynamical models
favored a driving period of order 104 yr to explain the observed jet
morphology.

Radio observations using the technique of phase-referencing allow for
ultrahigh-precision measurements of changes in the spatial location of a radio
source. Such observations have the great potential of directly measuring the
orbital motion of a jet-emitting SMBH (Sudou et al. 2003). This remarkable
technique was applied to the radio galaxy 3C 66B. Semi-periodic changes in the
radio VLBA core position at 2.3 GHz with a period of 1.05 yr were interpreted
as orbital motion in a binary SMBH system (Sudou et al. 2003). Since the model
requires a massive primary, part of the allowed parameter space was already
excluded by pulsar timing array (PTA) constraints. These constraints place an
upper limit of $M_{\rm BH,primary}<1.7\times 10^{9}$ M⊙ (Arzoumanian et al.
2020).

###  3.2 Semi-periodic light curves

If one of the SMBHs in a binary system is emitting a (radio) jet, then there
are several processes that produce a semi-periodic light-curve signal that
traces either the orbital evolution of the system or else is a sign of
precession induced by the second mass. Flux changes are then either true
changes of the intrinsic emission or they are artefacts of beaming due to a
jet with a systematically varying angle w.r.t our line of sight.

An unavoidable consequence of the presence of compact SMBBHs therefore is the
prediction that we should see semi-periodicities in light curves of at least a
fraction of the whole (radio) binary population. However, there are also
challenges when searching for periodicities in single light curves and/or
large data bases: On the one hand, we need densely sampled light curves that
cover at least several periods, since stochastic red noise variations can
mimic periodicities (Vaughan et al. 2016). On the other hand, true intrinsic
periodicities can be veiled by additional stochastic variability processes
which we know are omnipresent in accretion and jet systems.

Because of the great importance of identifying sub-parsec SMBBHs, many
dedicated searches are ongoing, and many candidates have been presented in the
last few years. Here, we would like to highlight two examples; the blazar OJ
287 (Sillanpää et al. 1996) and the flat-spectrum radio quasar (FSRQ) PG
1302–102 (Graham et al. 2015). OJ 287 is the best-studied and best-modelled
candidate to date for hosting a compact sub-parsec binary system and will be
further discussed in Sect. 4 and 5. PG 1302–102 was identified in a search for
periodic signals in light curves of 247000 quasars of the Catalina Transient
Survey data base (Graham et al. 2015). The system triggered multiple follow-up
observations and explorations of different variants of binary SMBH models
(e.g., D’Orazio et al. 2015, Kovačević et al. 2019, Saade et al. 2020, and
references therein; Fig. 3). Graham et al. (2015) reported a period of 5.2 yr.
Their model requires jet precession in an SMBBH with $<$0.01 pc separation.

The majority of sources with candidate semi-periodic light curves is radio-
loud. In radio-quiet systems, circumbinary disk simulations of mergers predict
(X-ray) emission periodically modulated at the orbital period (e.g., d’Ascoli
et al. 2018). Systems with optical continuum and line variability are further
discussed in Sect. 3.4.

Figure 3: Light curve (V magnitude, subtracted by mean value of V) of the
candidate SMBBH in PG 1302–102 and best-fit sinusoidal model, adapted from
Kovačević et al. 2019.

###  3.3 Tidal disruption event (TDE) light curves

Stars are tidally disrupted by SMBHs once the tidal forces of the black hole
exceed the self-gravity of the star. Part of the stellar material is then
accreted by the SMBH causing a luminous flare of electromagnetic radiation
that declines as $t^{-5/3}$ (Rees 1990). A few dozen TDEs have now been
reported (review by Komossa 2017), following their first discovery in X-rays
by the ROSAT mission.

The TDE lightcurves of single and binary SMBHs are characteristically
different. The binary model predicts characteristic dips and recoveries in a
TDE light curve when the second SMBH perturbs the stream of the stellar
material, temporarily interrupting and then restarting the accretion process
(Liu et al. 2009). This method of binary detection is of special interest, as
it probes the SMBBH population in quiescent, non-active galaxies, while most
other methods require at least one, or both, SMBHs to be active (an AGN) to
identify the binary system.

The first candidate SMBBH system identified from a TDE light curve is that of
SDSSJ1201+3003 (Liu et al. 2014). A model with $M_{\rm BH}=10^{6-7}$ M⊙ and
mass ratio $q=0.1$ at 0.6 milli-parsec separation reproduces the light curve
well.

###  3.4 Double-peaked, broad Balmer lines and their variability

A few percent of all quasars show broad, double-peaked emission lines from the
BLR. According to an early idea of Gaskell (1983), these could be the sign of
the presence of a binary SMBH, with each SMBH binding its own BLR. A key
prediction of the model is the Doppler-shift of each of the two line peaks as
the two SMBHs orbit each other, implying a changing red/blue shift of each
line peak on the timescale of years or decades. In the majority of the systems
monitored, most recently selected from the SDSS spectroscopic data base, the
predicted kinematic Doppler-shift was not observed, and warped disks in single
AGN or other mechanisms are the preferred interpretation (Doan et al. 2020). A
few candidate binary systems have remained. Their monitoring continues.

In a few cases, exceptional spectroscopic coverage already exists, spanning
decades. A very well monitored AGN interpreted as binary because of its
characteristic broad-line and continuum variability is NGC 4151. Based on an
outstanding 43 years of spectroscopic monitoring, Bon et al. (2012) reported
periodic variability in flux and in radial velocity of one BLR component that
they interpreted as shock waves generated by the supersonic motion of the
components through surrounding ISM. Their model requires an SMBBH with an
eccentric orbit and a period of 15.9 yr.

###  3.5 Additional methods

Many other methods have been suggested or have already been employed to
identify compact binary candidates (see also the recent review by de Rosa et
al. 2019). These include: UV/X-Ray deficits from truncated circumbinary disks
(Tanaka et al. 2012), periodic self-lensing and partial eclipses (D’Orazio &
di Stefano 2018, Ingram et al. 2021), acceleration of jet precession (Liu &
Chen 2007), characteristically variable and double-peaked/deformed Fe
K$\alpha$ lines (Yu & Lu 2001, McKernan et al. 2013), disappearance and
reappearance of AGN broad Balmer lines (Wang & Bon 2020), photocenter position
variability (Popović et al. 2012, Kovačević et al. 2020), astrometric orbital
motion tracking in the Gaia data base (D’Orazio & Loeb 2019) or with the Event
Horizon Telescope (Gómez, priv. com), magnetic field-line structure (Gold et
al. 2014), (radio)-jet polarimetry (Dey et al. 2021), and electromagnetic
signals contemporaneous with binary coalescence (Haiman 2017). Indirectly, the
detection of recoiling SMBHs also implies binary coalescences (Lousto &
Zlochower et al. 2011, Komossa et al. 2008). In recent years, PTAs have
started to place constraints on the population of binaries (Sesana et al.
2018) and were first used to constrain 3C 66B models (Sect. 3.1).

Figure 4: General-relativistic, precessing orbit of the secondary SMBH in the
SMBBH model of OJ 287, adopted from Dey et al. (2018) and Laine et al. (2020).
The primary SMBH is located at the origin with its accretion disk in the y = 0
plane. Flares arise due to the impacts of the secondary SMBH on the accretion
disk of the primary. But there is a delay (that can be calculated from the
model) between the actual impacts and the times when the flares become
visible. The black arrows point to the positions of the secondary SMBH at the
times when the impact flares become visible.

##  4 The case of OJ 287

The nearby blazar OJ 287 is the longest-studied and one of the best candidates
to date for hosting a compact SMBBH (reviews by Kidger 2007 and Dey et al.
2019), in a regime where GW emission already contributes to a measurable
shrinkage of the binary orbit (Valtonen et al. 2008, Dey et al. 2018, Laine et
al. 2020). We therefore review this system in some more detail. The unique
optical light curve of OJ 287 (e.g., Hudec et al. 2013) shows double-peaks
every $\sim$12 years that have been interpreted as arising from the orbital
motion of an SMBBH, with an orbital period on that order.

Different variants of binary scenarios of OJ 287 have been considered,
following the discovery that major optical outbursts of OJ 287 repeat
(Sillanpää et al. 1996). The best explored model by far explains the double
peaks as episodes where the secondary SMBH impacts the disk around the primary
twice during its $\sim$12 yr orbit (“impact flares” hereafter; Lehto &
Valtonen 1996, Valtonen et al. 2019). The most recent 4.5 order post-Newtonian
orbital modelling successfully reproduces the overall long-term light curve of
OJ 287 until 2019 (Valtonen et al. 2016, Dey et al. 2018, Laine et al. 2020,
and references therein). The model requires a compact SMBBH with a semi-major
axis of 9300 AU with a massive primary SMBH of $1.8\times 10^{10}$ M⊙ with
spin 0.38, and a secondary of $1.5\times 10^{8}$ M⊙. Because of the strong
general-relativistic orbital precession of the secondary, $\Delta\Phi$=38
deg/orbit, the impact flares are not always separated by 12 yr. Their
separation varies with time and in a predictable manner (Fig. 4).

In the SMBBH model, impact flares are triggered when the secondary SMBH
crosses the accretion disk twice during its orbit. The secondary’s impact
drives two supersonic bubbles of hot, optically thick gas from the disk. The
bubbles expand and cool. Once they become optically thin, they start emitting
and only then the flare becomes observable (see hydrodynamic simulations by
Ivanov et al. 1998). Impact flares were most recently reported in 2015 and
2019 (Valtonen et al. 2016, Laine et al. 2020). At such epochs, there is an
additional optical-IR emission component that may extend into soft X-rays, and
the total optical flux is less polarized (Valtonen et al. 2016, Ciprini et al.
2007). In addition to the impact flares, the model predicts “after-flares”
when the impact disturbance reaches the inner accretion disk (Sundelius et al.
1997) and triggers new jet activity, identified most recently with the bright
X-ray–UV–optical outburst in 2020 (Komossa et al. 2020).

Figure 5: Swift 0.3–10 keV X-ray light curve of OJ 287 since Dec. 2015,
including the two bright outbursts in 2016/17 and 2020 (Komossa et al. 2017,
2020). The majority of observations was obtained in the course of the MOMO
program.

##  5 The MOMO program

The MOMO program, for “Multiwavelength Observations and Modelling of OJ 287”
(Komossa et al. 2017, 2020, 2021) has an observational and a theoretical part.
The observational part consists of long-term flux and spectroscopic monitoring
and deep follow-up observations of OJ 287 at $>$13 different frequencies from
the radio to the X-ray band. The Neil Gehrels Swift observatory (Swift
hereafter) and the Effelsberg telescope play a central role. A few individual
observations are timed with the Event Horizon Telescope (EHT; Event Horizon
Telescope Collaboration et al. 2019) to obtain quasi-simultaneous
SEDs222Independent of the MOMO program, OJ 287 is a prime target of the EHT,
and has been observed annually with ALMA and GMVA since 2017, providing radio
VLBI observations of the twisted jet of OJ 287 at high resolution and
sensitivity (Gómez et al. 2021, in prep.). Such observations have the
potential of distinguishing between jet precession triggered by a binary or a
tilted precessing accretion disk.. MOMO was initiated in late 2015, with
$>$2000 data sets obtained so far. The program is the densest long-term
monitoring of OJ 287 involving X-rays and broad-band SEDs. The theoretical
part of the program aims at understanding aspects of accretion and jet physics
of the blazar central engine in general, and the binary SMBH in particular.
Some main findings of this ongoing project are summarized below.

Figure 6: Observed Swift SEDs of OJ 287 at selected epochs, including the
2016/17 and 2020 outbursts at peak, the March–April 2017 and 2018 near-EHT
epochs, and the 2020 low-state (Komossa et al. 2021). Optical–UV and X-ray
fluxes are correlated. The X-ray spectral steepening at high-states is due to
the increasing contribution of the synchrotron component(s).

Figure 7: Left: Comparison of all XMM-Newton spectra of OJ 287. A giant soft
X-ray excess is obvious in the 2020 spectrum (blue) obtained at the peak of
the outburst (Komossa et al. 2020). The XMM-Newton spectrum taken quasi-
simultaneous with the EHT observation in 2018, at intermediate flux level, is
marked in red. Right: Selected multifrequency Effelsberg radio spectra of OJ
287 between 2.6 and 40 GHz from the MOMO program.

(1) Our long-term Swift observations (Fig. 5; Komossa et al. 2017, 2020, 2021,
and 2021b in prep.) established OJ 287 as one of the most spectrally variable
blazars in the X-ray band (photon indices $\Gamma_{\rm x}$ = 1.5…3), changing
between inverse Compton emission at low-states, and a strong synchrotron
component at high-states.

(2) Two major X-ray–UV(–optical) outbursts were discovered with Swift in
2016/17 (Komossa et al. 2017) and in 2020 (Komossa et al. 2020)333The optical
outbursts were independently detected in ground-based monitoring campaigns
(e.g., Zola et al. 2020). The non-thermal nature of the outbursts was clearly
established based on multiple independent arguments: The exclusion of an
accretion-disk contribution because the X-rays varied faster than the light-
crossing time of the last stable orbit around the primary SMBH; the presence
of a radio outburst accompanying the X-ray-optical outburst; the close
correlations of fluxes in the Swift bands; and the high level of optical
polarization measured in independent projects (Komossa et al. 2020, and
references therein).

(3) The Swift multi-band coverage was enhanced around the dates EHT observed
OJ 287 in 2017 and 2018. Selected SEDs are shown in Fig. 6. In 2018, XMM-
Newton spectroscopy (Komossa et al. 2021; our Fig. 7) during the EHT campaign
revealed an intermediate-low flux and spectral state well described by a
combination of logarithmic parabolic power-law emission of synchrotron nature
and a flat ($\Gamma_{\rm x}=1.5$) IC component.

(4) A remarkable, giant soft X-ray excess (Fig. 7) of synchrotron origin was
discovered during the 2020 outburst of OJ 287 based on XMM-Newton and NuSTAR
observations (Komossa et al. 2020)444A similar soft emission component was
detected with Swift during the 2016/17 outburst (Komossa et al. 2017, 2020),
but we lack deeper XMM-Newton observations at that epoch.. NuSTAR also
revealed an additional and unusually soft emission component extending up to
$\sim$ 70 keV of unknown nature555 A mix of synchrotron and IC emission is a
possibility; another one is a temporary accretion-disk corona contribution
(even though there is no other optical–X-ray evidence for significant disk
emission during the outbursts).. Spectral evidence (at 2$\sigma$) for a
relativistically shifted iron absorption line in 2020 was seen with XMM-
Newton, however it needs independent confirmation in deeper future
observations that catch OJ 287 in the same state. The 2020 X-ray–optical
outburst was accompanied by a radio outburst (Fig. 7, right panel).

(5) The non-thermal 2020 outburst is consistent with an after-flare predicted
by the SMBBH model, where new jet activity is launched following a change in
the accretion rate as a consequence of the secondary’s disk impact (Komossa et
al. 2020).

The MOMO program will continue observations of OJ 287 as it nears its next
impact flare predicted by the SMBBH model (Dey et al. 2018), expected in 2022.

##  6 Summary and outlook

Binary SMBHs in all stages of their evolution are central to SMBH demographics
and galaxy evolution across cosmic times. The field has rapidly evolved in the
last decade, with many systems and candidates identified through
multiwavelength observations and orbital modelling, including candidate
evolved systems well beyond the final parsec and wide pairs in the early
stages of galaxy mergers. The fact that many of these systems are in nearby
galaxies (Tab. 1) implies that binaries should be common throughout the
universe. Binary SMBHs are most easily detectable electromagnetically, if at
least one or both SMBHs are active. However, an elusive population of binaries
could well exist at the cores of quiescent, inactive galaxies. Stellar tidal
disruption event lightcurves provide us with a unique tool of searching for
such a binary population. While the last few years have seen the first direct
detection of gravitational waves from stellar-mass black-hole binaries with
ground-based detectors, supermassive black-hole binaries are the loudest known
sources of GWs detectable with the future space-based gravitational-wave
interferometer LISA. Meanwhile, pulsar-timing arrays have greatly advanced in
recent years and have started to place constraints on the population of the
most massive binaries known. The EHT with its unprecedented spatial resolution
holds the promise of spatially resolving the small-separation SMBBH of OJ 287
for the first time. Among the population of sub-parsec binary SMBHs, the
blazar OJ 287 stands out as the longest-studied and best-studied candidate
which is already in a regime where gravitational-wave emission contributes
measurably to the orbital shrinkage. As a bright multimessenger source, OJ 287
is the target of an ongoing, dense monitoring program, MOMO. The program has
revealed high-amplitude outbursts interpreted in the context of the binary
model as after-flares. MOMO observations continue as OJ 287 nears its next
predicted impact flare.

Acknowledgements. SK would like to thank the organizers for their invitation
to give this review at the COST workshop on “The gravitational-wave Universe”
during the excellent XIX Serbian Astronomical Conference. We would like to
thank A.B. Kovačević for her permission to show her figure. This review is
based on published results obtained by the international community with a
multitude of observing facilities and surveys world-wide. Previously
unpublished data shown here for the first time were obtained with the Neil
Gehrels Swift observatory and the 100-m telescope of the Max-Planck-Institut
für Radioastronomie at Effelsberg.

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