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

TL;DR

The paper addresses how sub-parsec SMBH binaries can exhibit electromagnetic signatures when hot, advection-dominated flows coexist with cold thin disks. It introduces a three-disk binary-ADAF (BADAF) model that linearly combines the circumbinary disk and two circum-single disks, treating cold and hot components with appropriate emission mechanisms and an electron-heating parameter, to predict full spectral energy distributions across the radio to gamma-ray bands. The study identifies where periodic variability at the binary orbital period is most likely to appear (primarily in thermal thin-disk emission and, for some configurations, low-frequency synchrotron) and discusses Doppler/lensing effects as additional variability channels. It applies the framework to the SMBHB candidate PG1302-102, showing that Region II BADAF can reproduce its broadband SED when supplemented with illustrative jet and dust components, and highlights the model’s utility for guiding multi-wavelength follow-up and multi-messenger efforts while outlining limitations and future directions for more self-consistent, non-linear treatments. Overall, the BADAF approach provides a coherent, testable pathway to interpret electromagnetic signatures of SMBHBs and to constrain their orbital and accretion properties in the era of time-domain and multi-messenger astronomy.

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.

Figures (7)

• Figure 1: Critical accretion rates evaluated at the inner edge of a circumbinary disk ($r=2a_b$) for binaries with mass $M$ and period $P_b$ (and viscosity $\alpha = 0.3\alpha_{0.3}$). Below the indicated $\dot m_{\rm crit}$, hot advection-dominated solutions are available. Binaries that would contribute to the GWB at redshift $z=0.3$ are highlighted in blue. Grey pluses represent fiducial binary systems used throughout.
• Figure 2: Characterization of multi-component accretion states. (Top) Schematics illustrating available combinations of thin disk (blue) and ADAF (red) components. (Bottom-left) Critical accretion rates for the full binary ($\dot m_{0, {\rm crit}}$; solid), primary ($\dot m_{1, {\rm crit}}$; dashed), and secondary ($\dot m_{2, {\rm crit}}$; dot-dashed) as a function of the binary mass ratio $1/q$ and mass supply rate $\dot m_0$. The Roman numerals indicate the regions of $\dot m_0 - q$ space corresponding to different accretion geometries as illustrated in the schematics above. The vertical grey band shows our fiducial choice of mass ratio for Section \ref{['S:spectral-characteristics']}. (Right) The colormap shows the secondary accretion rate $\dot m_2$, and the thin white band indicates when the secondary starts accreting above its Eddington limit (top, blue regions) under the assumed form of $\lambda(q)$. $\dot m_2$ only ever mildly exceeds its Eddington limit, such that this effect is not significant for our analysis (see text).
• Figure 3: Full broadband SEDs for the fiducial binaries from our multi-component three-disk model: (Left) $M = 10^7 M_\odot$, $P_b = 6\,\unit{months}$, and (Right) $M = 10^{10} M_\odot$, $P_b = 10\,\unit{years}$. Dashed lines are for a period a factor of six shorter (a month and $\sim 1.5$yr, respectively).
• Figure 4: SED decomposition into contributions from the circumbinary disk (cbd), the primary minidisk (md1), and the secondary minidisk (md2) for the binary with $M = 10^{10} M_\odot$ and $P_b = 10\unit{yr}$. The solid grey line shows the total SED. The accretion rate and associated Region are indicated in the top left of each panel.
• Figure 5: Characteristic cooling profiles for Bremsstrahlung, synchrotron, and inverse Compton emission from the ADAF self-similar solutions for a $10^7 M_\odot$ binary with a six month period (upper panel) and a $10^{10} M_\odot$ binary with a ten year period (lower panel) as a function of radius in units of binary semi-major axes. Both panels are for an outer mass supply rate $5 \times 10^{-3} \dot{M}_{\rm Edd}$. Dashed lines show an equivalent binary with one-sixth the period. The grey region shows typical scales of the circumbinary cavity such that regions to its left are generally part of the circum-single flows (mds) and to the right part of the circumbinary flow (cbd). The solid grey line illustrates the characteristic inflow time for hot, advection-dominated solutions, which we regard as a lower limit (see text).