Mergers of black hole binaries driven by misaligned circumbinary discs

With hydrodynamical simulations we examine the evolution of a highly misaligned circumbinary disc around a black hole binary including the effects of general relativity. We show that a disc mass of just a few percent of the binary mass can significantly increase the binary eccentricity through von-Zeipel--Kozai-Lidov (ZKL) like oscillations provided that the disc lifetime is longer than the ZKL oscillation timescale. The disc begins as a relatively narrow ring of material far from the binary and spreads radially. When the binary becomes highly eccentric, disc breaking forms an inner disc ring that quickly aligns to polar. The polar ring drives fast retrograde apsidal precession of the binary that weakens the ZKL effect. This allows the binary eccentricity to remain at a high level and may significantly shorten the black hole merger time. The mechanism requires the initial disc inclination relative to the binary to be closer to retrograde than to prograde.