The role of radiation pressure in accreting massive black hole binaries

¹ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
² INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
³ Dipartimento di Fisica “A. Pontremoli”, Università degli Studi di Milano, Via Giovanni Celoria 16, 20134 Milano, Italy
⁴ DiSAT, Università degli Studi dell’Insubria, via Valleggio 11, I-22100 Como, Italy

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 25 August 2025
Accepted: 3 February 2026

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 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 10⁶ 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.