Differential Interferometric Signatures of Close Binaries of
  Supermassive Black Holes in Active Galactic Nuclei
  In the present paper, we explore opportunities of applying the GRAVITY at the
Very Large Telescope Interferometry (VLTI) with unprecedented spatial
resolution to identify close binaries of supermassive black holes (CB-SMBHs) in
active galactic nuclei (AGNs). Each SMBH is assumed to be separately surrounded
by their own broad-line regions (BLRs) composed of clouds with virialized
motion. Composition of the binary orbital motion and the virial motion of
clouds in each BLR determines the projected velocity fields and hence
differential phase curves, which are obviously different from that of a single
BLR. We calculate emission line profiles and differential phase curves of
CB-SMBHs for the GRAVITY. For the simplest case where angular momentums of two
BLRs and orbital motion are parallel, a phase plateau generally appears in the
phase curves. For other combinations of the angular momentum, the plateau is
replaced by new peaks and valleys variously depending on the situations. Given
a combination, phase curves are also sensitive to changes of parameters of
CB-SMBHs. All these features are easily distinguished from the well-known
$S$-shaped phase curves of a single BLR so that the GRAVITY is expected to
reveal signals of CB-SMBH from candidates of AGNs. With joint analysis of
observations of reverberation mapping campaigns, we can reliably identify
CB-SMBHs, and measure their orbital parameters in the meanwhile. This
independent measurement of the orbital parameters also has implications to
analysis of Pulsar Timing Array (PTA) observations for properties of
low-frequency gravitational waves in future.
