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

Abstract

Binary black hole systems (BBHs) have become a vivid reality in astrophysics as stellar-mass black hole mergers are now detected through their related gravitational wave emission during the merger stage. If many studies were recently dedicated to the last stages of BBH where black holes are surrounded by a circumbinary disk (CBD), the structure of these systems prior to the formation of the CBD remains mostly unexplored. The aim of the present article is to investigate the potential modifications induced by the presence of a secondary black hole onto the structure of the accretion disk surrounding the primary black hole. We performed 2D classical hydrodynamical simulations of an accretion disk surrounding the primary black hole while taking into account all gravitational effects induced by both the primary black hole and the secondary black hole orbiting on circular orbits around the center of mass of the system.We report three main effects of the presence of a secondary black hole orbiting a circular orbit beyond the outer edge of the accretion disk: 1/ the outer radius of the accretion disk is significantly reduced and its ratio to the black hole separation is directly linked to only the mass ratio of the black holes; 2/ two spiral arms are visible in the gas density structure of the disk and 3/ the outer edge of the accretion disk exhibits an elliptical shape that mainly depends on the mass ratio of the black holes. Our results show that an accretion disk 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, directly linked to the binary separation and mass ratio, has therefore the potential to help in the search and identification of BBH in the pre-CBD stage.