Lense-thirring Precession of Accretion Disks around Compact Objects

Misaligned accretion disks surrounding rotating compact objects experience a torque due to the LenseThirring effect, which leads to precession of the inner disk. It has been suggested that this effect could be responsible for some low frequency Quasi-Periodic Oscillations observed in the X-ray lightcurves of neutron star and galactic black hole systems. We investigate this possibility via time-dependent calculations of the response of the inner disk to impulsive perturbations for both Newtonian point mass and Paczynski-Wiita potentials, and compare the results to the predictions of the linearized twisted accretion disk equations. For most of a wide range of disk models that we have considered, the combination of differential precession and viscosity causes the warps to decay extremely rapidly. Moreover, at least for relatively slowly rotating objects, linear calculations in a Newtonian point mass potential provide a good measure of the damping rate, provided only that the timescale for precession is much shorter than the viscous time in the inner disk. The typically rapid decay rates suggest that coherent precession of a fluid disk would not be observable, though it remains possible that the damping rate of warp in the disk could be low enough to permit weakly coherent signals from Lense-Thirring precession. Subject headings: accretion, accretion disks — hydrodynamics — black hole physics – relativity – stars: neutron – X-rays: general Animation of warp evolution is available at: http://www.cita.utoronto.ca/∼armitage/lense thirring.html