Gravitational waves (GW) in the nano-Hz domain are expected to be radiated by
close-binaries of supermassive black holes (CB-SMBHs; components bound in a
Keplerian binary at mutual distance less than ~ 0.1 pc), which are relicts
of galaxy mergers and anticipated to be measured via the Pulsar Timing Array
(PTA) technique. The challenge of present CB-SMBH investigations is that
their signatures are elusive and not easily disentangled from a single SMBH.
PTAs will typically have a glimpse of an early portion of the binary
inspiral to catch the frequency evolution of the binary only with sufficiently
high mass and initially high eccentricity. Thus, we have to make use of
electromagnetic observations to determine orbital parameters of CB-SMBHs and
test nano-Hz GW properties. The 2D reverberation mapping (RM) is a powerful
tool for probing kinematics and geometry of ionized gas in the SMBHs (single
or binary) vicinity, yet it can lose information due to projection on the
line of sight of the observer. Nevertheless, spectroastrometry with AMBER,
GRAVITY, and successors can provide an independent measurement of the
emitting region's size, geometry, and kinematics. These two techniques
combined can resolve CB-SMBHs. In this review, we focus on RM and
spectroastrometry observational signatures of CB-SMBHs with non-zero
eccentricity from recent simulations with particular attention to recent
developments and open issues.
AGNs on the Move: A Search for Off-nuclear AGNs from Recoiling Supermassive Black Holes and Ongoing Galaxy Mergers with the Zwicky Transient Facility
