Speaker
Description
Spin configuration of supermassive black-hole binaries affect gravitational waves at post-Newtonian level, making it observable by the future LISA space mission. Spin alignment is of significant relevance not only for black-hole recoils, whose kick magnitude increases with binary misalignment and can be greater than galaxies' escape velocity, but also as a possible discriminant between gas-poor and gas-rich binary environments. Gas-rich galaxies are able to provide an accretion channel thanks to the presence of a circumbinary disc, consequentially producing a massive circum-black-hole disc which drives alignment through the Bardeen-Petterson effect. In gas-poor environments, such a channel does not exist and the binary is therefore expected to merge in a misaligned configuration.
One-dimensional numerical integrations of the disc conservation equations have already attempted to reproduce the shape and evolution of the system in a gas-rich environment. However, such integrations assume an infinite disc, fixed at its outer boundary, an approximation which inherently prevents disc evolution. We propose a new non-local boundary-value problem formulation, in which we relax this assumption by globally constraining the disc total angular momentum instead of fixing its specific value at the outer boundary. Our approach is able to reproduce both massive discs and systems where the disc angular momentum is comparable to that of the black hole, while also incorporating non-linear viscosities. We investigate whether the Bardeen–Petterson effect is sufficient to drive alignment or counter-alignment in the latter regime, or whether the disc breaks before such a configuration is achieved. If such low-angular-momentum discs can exist, this could imply that binaries in gas-poor environments may also achieve aligned (or counter-aligned) configurations at merger. Although less detailed than full hydrodynamical simulations, our framework has the advantage of being significantly faster and cost-effective, allowing for an efficient exploration of the parameter space and quicker prediction of the end-state of the system.
| Parallel session | Gravitational Waves from Binary Systems |
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