The role of radiation pressure in accreting massive black hole binaries

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 was therefore allowed to change through viscous heating, black-body cooling, and self-gravity. We made a significant decision to also include the contribution of radiation pressure in the simulations. We modelled binaries with a total mass of 10^6 , M_⊙, eccentricities of e=0,0.45,0.9, and mass ratios of 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 and 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.