Radiation pressure role in accreting massive black hole binaries

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.