Close binaries of supermassive black holes ( CB-SMBHs ) with separations of \lesssim 0.1 pc as the final stage of galaxy mergers are sources of low frequency gravitational waves ( GW ) , however , they are still elusive observationally because they are not spatially resolved .
Fortunately , reverberation as echoes of broad emission lines to ionizing continuum conveys invaluable information of the dynamics of broad-line regions ( BLRs ) governed by supermassive black holes in the central regions of active galactic nuclei ( AGNs ) .
In this paper , we demonstrate how to composite the hybrid 2-dimensional transfer functions of binary BLRs around the CB-SMBHs in AGNs , providing an opportunity of identifying them from reverberation mapping ( RM ) data .
It is found that there are variation-coupling effects in the transfer functions , arising from the coupling of CB-SMBH light curves in the Fourier space .
We provide semi-analytical formulations of the transfer functions for kinematic maps of the gas .
For cases with the simplest variation-coupling effects , we make calculations for several BLR models and reveal significant distinctions from those of single active black holes .
In principle , the difference is caused by the orbital motion of the CB-SMBH systems .
In order to search for CB-SMBHs in time-domain space , selection of target candidates should focus on local AGNs with H \beta double-peaked profiles and weaker near-infrared emission .
High-fidelity RM-campaigns of monitoring the targets in future will provide opportunities to reveal these kinematic signatures of the CB-SMBHs and hence for measurements of their orbital parameters .