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

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 with a total mass of 2 × 10 7 M ⊙ on a circular orbit, separated by 100 GM tot / c 2 . The inclusion of radiation leads to a denser, thinner, and more filamentary disk compared to nonradiative magnetohydrodynamic simulation, primarily due to reduced pressure support and an altered equation of state. The RMHD disk also features ∼3 times lower accretion rate ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>≈</mml:mo> <mml:mn>0.15</mml:mn> <mml:mspace width="0.25em"/> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">Edd</mml:mi> </mml:mrow> </mml:msub> </mml:math> ), 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 Laser Interferometer Space Antenna gravitational-wave precursors, and for heavier binaries that are Pulsar Timing Array sources.