Hot, cold, and multi-component accretion flows around supermassive black hole binaries

TL;DR

This paper presents a model for supermassive black hole binaries with multi-component accretion flows, explaining their spectral properties and variability, and applies it to a candidate system, aiding future observational efforts.

Contribution

It introduces a three-disk spectral model for SMBHBs with hot and cold accretion components, providing new insights into their observational signatures and variability.

Findings

Binary accretion can explain many properties of standard AGN.

Variability at the binary period is most likely in thermal and low-frequency synchrotron emission.

A mixed accretion state can fit the observed spectrum of PG1302-102.

Abstract

We develop a model for supermassive black hole binaries (SMBHBs) accreting below their Eddington limit, focusing on systems where hot, advection-dominated flows become viable. We specifically explore the spectral appearance of multi-component accretion flows where the solution can independently transition between cold, thin disks and hot, advection-dominated torii depending on the local accretion rate. Using a three-disk model, we compute spectral energy distributions for four possible accretion configurations and assess their observational signatures, including which frequencies might reflect variability at the binary orbital period. The spectral modeling reveals that binary accretion can self-consistently account for many of the properties of standard AGN, while the variability analysis shows that hydrodynamic modulation at the binary period is most likely in the thermal emission and low-frequency synchrotron components. Doppler boosting of emitting material bound to a single binary component would also induce periodic variability. We apply our model to the SMBHB candidate PG1302-102 and demonstrate that a mixed-component accretion state (plus a jet feature) can self-consistently capture the observed broadband spectrum. Our model offers a framework for interpreting candidate SMBHBs and motivates future multi-wavelength follow-up of potential multi-messenger sources, as well as more detailed future modeling of multi-component binary accretion.