Rebecca Nealon, Daniel J. Price, Chris J. Nixon

full text pdf, accepted to MNRAS 06 January 2015

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Abstract

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 α-discs subject to Lense-Thirring precession, focussing on the bending wave regime where the disc viscosity is smaller than the aspect ratio α ≤ 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 (α ≥ 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.

Movies Created using SPLASH. See also Rebecca Nealon's page.	Movies and images (c) 2015 Rebecca Nealon, released under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 License. [directory listing of all movies here]
	Cross section of the disc, showing development of oscillations in the tilt angle between the disc and the black hole
	Evolution of an accretion disc initially inclined to the black hole spin by 120 degrees
	Evolution of an accretion disc initially inclined to the black hole spin by 30 degrees
	Evolution of an accretion disc initially inclined to the black hole spin by 60 degrees
	Evolution of an accretion disc initially inclined to the black hole spin by 90 degrees
	Evolution of the angle between the disc plane and the spin axis of the black hole, at 3 different resolutions