Kinematic Signatures of Reverberation Mapping of Close Binaries of Supermassive Black Holes in Active Galactic Nuclei

Close binaries of supermassive black holes (CB-SMBHs) with separations of ≲0.1 pc as the final stage of galaxy mergers are sources of low-frequency gravitational waves; 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 on the dynamics of broad-line regions (BLRs) governed by SMBHs in the central regions of active galactic nuclei (AGNs). In this paper, we demonstrate how to compose the hybrid two-dimensional transfer functions (TFs) of binary BLRs around the CB-SMBHs in AGNs, providing an opportunity for identifying them from reverberation mapping (RM) data. It is found that there are variation-coupling effects in the TFs arising from the coupling of CB-SMBH light curves in the Fourier space. We provide semi-analytical formulations of the TFs 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 Hβ 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 measurements of their orbital parameters.