Kinematic signatures of reverberation mapping of close binaries of supermassive black holes in active galactic nuclei III. The case of elliptical orbits

TL;DR

This study models the kinematic signatures of elliptical supermassive black hole binaries in active galactic nuclei using two-dimensional transfer functions, revealing distinct features that can aid in their detection and characterization.

Contribution

First to compute 2D transfer functions for elliptical SMBH binaries, demonstrating their potential to identify and analyze such systems in AGN spectra.

Findings

Elliptical SMBH systems produce distinct 2DTF maps.

Line profiles vary with orbital phase and resemble observed features.

2DTF can help distinguish SMBH binaries from other models.

Abstract

An intense search for kinematic signatures of reverberation mapping of close binary SMBH (SMBBH) is currently ongoing. Elliptical SMBBH systems (i.e. both orbits and disc-like broad-line regions (BLR) are elliptic) have not been assessed in two-dimensional transfer functions (2DTF) studies. We aim to numerically infer such a 2DTF because the geometry of the unresolved region is imprinted on their optical emission. Through this, we determine their specific kinematical signatures. We simulated the geometry and kinematics of SMBBH whose components are on elliptical orbits. Each SMBH had a disc-like elliptical BLR. The SMBHs were active and orbited each other tightly at a subparsec distance. Here we calculate for the first time 2DTF, as defined in the velocity-time delay plane, for several elliptical configurations of SMBBH orbits and their BLRs. We find that these very complex configurations are clearly resolved in maps. These results are distinct from those obtained from circular and disc-wind geometry. We calculate the expected line variability for all SMBBH configurations. We show that the line shapes are influenced by the orbital phase of the SMBBH. Some line profiles resemble observed profiles, but they can also be much deformed to look like those from the disc-wind model. First, our results imply that using our 2DTF, we can detect and quantify kinematic signatures of elliptical SMBBH. Second, the calculated expected line profiles share some intriguing similarities with observed profiles, but also with some profiles that are synthesised in disc-wind models. To overcome the non-uniqueness of the spectral line shapes as markers of SMBBH, they must be accompanied with 2DTF.