Sculpting the outer edge of accretion discs in pre-circumbinary binary black hole systems

Context. Binary black hole (BBH) systems have become a vivid reality in astrophysics as stellar-mass black hole mergers can now be detected through their related gravitational wave emission during the merger stage. While there have been a number of recent studies dedicated to the last stages of BBHs, where black holes are surrounded by a circumbinary disc (CBD), the structure of these systems prior to the formation of the CBD remains mostly unexplored. Aims. The aim of this paper is to investigate the potential modifications induced by the presence of a secondary black hole onto the structure of the accretion disc surrounding the primary black hole. Identifying potential specific features of the accretion flow in pre-circumbinary BBHs may help in identifying such systems through their electromagnetic emission. Methods. We performed 2D classical hydrodynamical simulations of an accretion disc surrounding the primary black hole. We took into account all gravitational effects induced by both the primary black hole and the secondary black hole on circular orbits around the system’s centre of mass. Results. We report three main effects of the presence of a secondary black hole orbiting a circular orbit beyond the outer edge of the accretion disc: (1) the outer radius of the accretion disc is significantly reduced and its ratio to the black hole separation is directly linked solely to the mass ratio of the black holes; (2) two spiral arms are visible in the gas density structure of the disc; and (3) the outer edge of the accretion disc exhibits an elliptical shape that mainly depends on the mass ratio of the black holes. Conclusions. Our results show that an accretion disc orbiting a primary black hole in a pre-CBD BBH exhibits specific features induced by the gravitational force generated by the presence of a secondary black hole beyond its outer edge. Such features, which are directly linked to the binary separation and mass ratio, hold the potential to aid in the search and identification of BBHs in the pre-CBD stage.