Hydrodynamic simulations of the Bardeen-Petterson effect

We present SPH simulations of accretion discs in orbit about rotating compact objects, such as black holes and neutron stars, and study the structure of warped discs produced by the Bardeen-Petterson effect. We calculate the transition radius out to which the disc specific angular momentum vector is aligned with that of the black hole. We focus on the parameter regime where the warp dynamics are controlled by bending wave propagation, but also consider models in which warps are subject to diffusion rather than wave transport, and are able to consider the fully non-linear regime. Because of hydrodynamic or pressure effects, for the parameter range investigated, the transition radius is always found to be much smaller than that obtained by Bardeen & Petterson. For discs with mid-plane Mach numbers of ∼10, the transition occurs between 10 and 16R+ (gravitational radii), whereas for a Mach number of ∼30 it occurs at around 30R+. A thicker disc with a Mach number of 5 is found to produce no discernible warped structure. The rate of black hole-disc alignment is found to be consistent with the ideas of Rees, with the alignment torque behaving as if it arises from the accreted material transferring its misaligned component of angular momentum at the larger transition radius of Bardeen & Petterson. The inclusion of Einstein precession in the calculations modified both the warped disc structure and, consistent with linear analysis, produced an increased alignment rate by up to a factor of 4 because of the effect that the non-Keplerian potential has on the propagation of warps.