Radiation pressure role in accreting massive black hole binaries

TL;DR

This study explores how radiation pressure influences the structure and evolution of circumbinary discs around massive black hole binaries, revealing that radiation pressure makes the disc thinner and colder, affecting accretion and orbital dynamics.

Contribution

The paper introduces the first 3D hydrodynamic simulations including radiation pressure effects on circumbinary discs around massive black hole binaries.

Findings

Radiation pressure makes the disc thinner and colder.

It reduces accretion rates onto the binary.

It suppresses cavity eccentricity growth and precession.

Abstract

We investigate the impact of radiation pressure on the circumbinary discs surrounding accreting massive black hole binaries (MBHBs) at milli-parsec separations, using 3D hyper-Lagrangian resolution hydrodynamic simulations. The circumbinary discs in our simulations evolve under an adiabatic equation of state. The gas temperature is therefore allowed to change through viscous heating, black-body cooling and self-gravity. We take a significant step further by including the contribution of radiation pressure in the simulations. We model binaries with a total mass of $10^6 \, M_{\odot}$, eccentricities $e=0,0.45,0.9$ and mass ratios $q= 0.7, 1$. We find that the radiation pressure significantly alters the vertical and thermal structure of the disc, resulting in a geometrically thinner, therefore colder configuration. This leads to a reduced accretion rate onto the binary and suppresses cavity eccentricity growth and precession in circular equal mass binaries. The binary eccentricity remains approximately constant, while the semi-major axis decreases over time due to net negative torque, regardless of the initial binary orbital parameters.