Kinematic signatures of reverberation mapping of close binaries of supermassive black holes in active galactic nuclei

TL;DR

This paper explores how reverberation mapping can identify close binary supermassive black holes in active galactic nuclei by analyzing unique kinematic signatures in transfer functions derived from emission line data.

Contribution

It introduces a semi-analytical framework for modeling 2D transfer functions of binary black hole systems, revealing distinctive signatures caused by their orbital motion.

Findings

Transfer functions show variation-coupling effects unique to binary systems.

Significant differences from single black hole models are caused by orbital motion.

Target selection should focus on AGNs with specific spectral features for effective detection.

Abstract

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.