The dynamics of a twisted disc formed after the tidal disruption of a star by a rotating black hole

Astro Space Centre, P.N. Lebedev Physical Institute, 4/32 Profsoyuznaya Street, Moscow, 117810, Russia

We consider misaligned accretion discs formed after tidal disruption events occurring when a star encounters a supermassive rotating black hole. We use the linear theory of warped accretion discs to find the disc shape when the stream produced by the disrupted star provides a source of mass and angular momentum that is misaligned with that of the black hole. The evolution of the surface density and aspect ratio is found from a 1D vertically averaged model. We extend previous work which assumed a quasi-stationary disc to allow unrestricted dynamical propagation of disc tilt and twist through time-dependent backgrounds. We consider a smaller value of the viscosity parameter, α = 0.01, finding the dynamics varies significantly. At early times the disc inclination is found to be nearly uniform at small radii where the aspect ratio is large. However, since torques arise from the Lense-Thirring effect and the stream there is non-uniform precession. We propose a simple model for this requiring only the background surface density and aspect ratio. At these times the α ∼ 0.01 disc exhibits a new feature. An inclined hot inner region joins an outer low inclination cool region via a thin transition front propagating outwards with a speed exceeding that of bending waves in the cool region. These waves accumulate where the propagation speeds match producing an inclination spike separating inner and outer discs. At late times a sequence of quasi-stationary configurations approximates disc shapes at small radii. We discuss observational implications of our results.

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