Radiation Magnetohydrodynamic Simulation of sub-Eddington Circumbinary Disk around an Equal-mass Massive Black Hole Binary

TL;DR

This study uses advanced 3D radiation magnetohydrodynamic simulations to explore the structure and observational signatures of circumbinary disks around equal-mass massive black hole binaries, highlighting radiation's significant impact.

Contribution

First RMHD simulation of a sub-Eddington circumbinary disk around an equal-mass massive black hole binary, revealing radiation's effects on disk structure and variability.

Findings

Radiation leads to denser, thinner, more filamentary disks.

Accretion rate is about 0.15 times the Eddington rate.

Binary-induced variability is observable in optical/UV bands.

Abstract

We present the first three-dimensional radiation magnetohydrodynamic (RMHD) simulation of a sub-Eddington circumbinary disk (CBD) around an equal-mass massive black hole binary (MBHB) with a total mass of $2\,\times\,10^7\,M_{\odot}$ on a circular orbit, separated by 100$\,GM_{\rm tot}/c^2$. The inclusion of radiation leads to a denser, thinner, and more filamentary disk compared to non-radiative magnetohydrodynamic simulation, primarily due to reduced pressure support and an altered equation of state. The RMHD disk also features $\sim 3$ times lower accretion rate ($\approx 0.15\,\dot{M}_{\rm Edd}$), weaker accretion streams and a less pronounced overdensity (a.k.a., ``lump") at the inner edge. Our analysis of the light curves and thermal spectra reveals that the variability induced by the binary-CBD interaction is distinguishable in the optical/UV band, where CBD shines at about $1\%$ of the Eddington luminosity. These findings underscore the crucial role of radiation on the structure and observational properties of CBDs around massive black hole binaries and have implications for detecting electromagnetic counterparts to LISA gravitational wave precursors, and for heavier binaries that are Pulsar Timing Array sources.