Chris Nixon

Tearing up the disc: misaligned accretion on to a binary

Support for this work was provided by NAS through the Einstein Fellowship Programme, grant PF2130098. Research in theoretical astrophysics at Leicester is supported by an STFC Consolidated Grant. We used SPLASH (Price 2007) for the visualization. The calculations for this paper were performed on the Complexity node of the DiRAC2 HPC Facility which is jointly funded by STFC, the department of Business Innovation and Skills and the University of Leicester.

Retrograde accretion and merging supermassive black holes

We investigate whether a circumbinary gas disc can coalesce a supermassive black hole binary system in the centre of a galaxy. This is known to be problematic for a prograde disc. We show that in contrast, interaction with a retrograde circumbinary disc is considerably more effective in shrinking the binary because there are no orbital resonances. The binary directly absorbs negative angular momentum from the circumbinary disc by capturing gas into a disc around the secondary black hole, or discs around both holes if the binary mass ratio is close to unity. In many cases the binary orbit becomes eccentric, shortening the pericentre distance as the eccentricity grows. In all cases the binary coalesces once it has absorbed the angular momentum of a gas mass comparable to that of the secondary black hole. Importantly, this conclusion is unaffected even if the gas inflow rate through the disc is formally super-Eddington for either hole. The coalescence time-scale is therefore always ∼ M2/M. where M 2 is the secondary black hole mass and M the inflow rate through the circumbinary disc.

Tearing up the disk: How black holes accrete

We show that in realistic cases of accretion in active galactic nuclei or stellar-mass X-ray binaries, the Lense-Thirring effect breaks the central regions of tilted accretion discs around spinning black holes into a set of distinct planes with only tenuous flows connecting them. If the original misalignment of the outer disc to the spin axis of the hole is

On the Bardeen-Petterson effect in black hole accretion discs

45^{\circ} \lesssim \theta \lesssim 135^{\circ}

Broken discs: warp propagation in accretion discs

, as in

Stable counteralignment of a circumbinary disc

\sim 70

The Kozai-Lidov Mechanism in Hydrodynamical Disks

% of randomly oriented accretion events, the continued precession of these discs sets up partially counter-rotating gas flows. This drives rapid infall as angular momentum is cancelled and gas attempts to circularize at smaller radii. Disc breaking close to the black hole leads to direct dynamical accretion, while breaking further out can drive gas down to scales where it can accrete rapidly. For smaller tilt angles breaking can still occur, and may lead to other observable phenomena such as QPOs. For such effects not to appear, the black hole spin must in practice be negligibly small, or be almost precisely aligned with the disc. Qualitatively similar results hold for any accretion disc subject to a forced differential precession, such as an external disc around a misaligned black hole binary.

Rapid AGN accretion from counter‐rotating discs

We investigate the effect of black hole spin on warped or misaligned accretion discs — in particular i) whether or not the inner disc edge aligns with the black hole spin and ii) whether the disc can maintain a smooth transition between an aligned inner disc and a misaligned outer disc, known as the Bardeen-Petterson effect. We employ high resolution 3D smoothed particle hydrodynamics simulations of -discs subject to Lense-Thirring precession, focussing on the bending wave regime where the disc viscosity is smaller than the aspect ratio . H=R. We first address the controversy in the literature regarding possible steady-state oscillations of the tilt close to the black hole. We successfully recover such oscillations in 3D at both small and moderate inclinations (. 15 ), provided both Lense-Thirring and Einstein precession are present, sufficient resolution is employed, and provided the disc is not so thick so as to simply accrete misaligned. Second, we find that discs inclined by more than a few degrees in general steepen and break rather than maintain a smooth transition, again in contrast to previous findings, but only once the disc scale height is adequately resolved. Finally, we find that when the disc plane is misaligned to the black hole spin by a large angle, the disc ‘tears’ into discrete rings which precess effectively independently and cause rapid accretion, consistent with previous findings in the diffusive regime ( & H=R). Thus misalignment between the disc and the spin axis of the black hole provides a robust mechanism for growing black holes quickly, regardless of whether the disc is thick or thin.

Rapid AGN accretion from counter-rotating discs

We simulate the viscous evolution of an accretion disc around a spinning black hole. In general, any such disc is misaligned, and warped by the Lense–Thirring effect. Unlike previous studies, we use effective viscosities constrained to be consistent with the internal fluid dynamics of the disc. We find that non-linear fluid effects, which reduce the effective viscosities in warped regions, can promote breaking of the disc into two distinct planes. This occurs when the Shakura & Sunyaev dimensionless viscosity parameter α is ≲0.3 and the initial angle of misalignment between the disc and hole is ≳45°. The break can be a long-lived feature, propagating outwards in the disc on the usual alignment time-scale, after which the disc is fully co-aligned or counter-aligned with the hole. Such a break in the disc may be significant in systems where we know the inclination of the outer accretion disc to the line of sight, such as some X-ray binaries: the inner disc, and so any jets, may be noticeably misaligned with respect to the orbital plane.

Planet–disc evolution and the formation of Kozai–Lidov planets

In general, when gas accretes on to a supermassive black hole binary it is likely to have no prior knowledge of the binary angular momentum. Therefore a circumbinary disc forms with a random inclination angle, theta, to the binary. It is known that for theta 90 degrees the disc wholly counteraligns if it satisfies cos(theta) 90 degrees and this criterion is not satisfied the same disc may counteralign its inner regions and, on longer timescales, coalign its outer regions. I show that for typical disc parameters, describing an accretion event on to a supermassive black hole binary, a misaligned circumbinary disc is likely to wholly co-- or counter--align with the binary plane. This is because the binary angular momentum dominates the disc angular momentum. However with extreme parameters (binary mass ratio M_2/M_1 << 1 or binary eccentricity e ~ 1) the same disc may simultaneously co- and counter-align. It is known that coplanar prograde circumbinary discs are stable. I show that coplanar retrograde circumbinary discs are also stable. A chaotic accretion event on to an SMBH binary will therefore result in a coplanar circumbinary disc that is either prograde or retrograde with respect to the binary plane.