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Abstract

The inspiral of supermassive black-hole binaries in gas-rich environment is
driven by the presence of an accretion disc and viscous interactions tend to
align the spin of the black holes with the orbital angular momentum of the
disc. Recent work introduced a new iterative approach to describe the alignment
process and the resulting non-linear evolution of the surrounding warped
accretion disc. Their model predicted that black-hole spins reach either full
alignment or a critical obliquity where solutions to the warp equations cease
to exist. In this paper, we show that this critical region corresponds to the
disc breaking phenomenon, where the disc is disrupted into two or more discrete
sections. We use 3D hydrodynamical simulations to (i) recover the predictions
of the semi-analytic model and (ii) unveil a richer phenomenology where the
disc exhibits either unsuccessful, single and multiple breaks. We additionally
identify hydrodynamic effects such as spiral arms that are able to stabilise
the disc against breaking beyond criticality. Our results show that when disc
breaking occurs, the ability of black holes and disc to align is compromised
and in some cases even prevented as the binary inspirals.