Disk tearing leads to High and Low frequency quasi-periodic oscillations in a GRMHD simulation of a tilted accretion disk

Black hole X-ray binaries (BHXRBs) display a wide range of variability phenomena, from large-scale spectral state changes to broadband variability and quasi periodic oscillations (QPOs), however the physical mechanisms driving this variability are not well understood. QPOs can be used as a powerful diagnostic tool of black hole accretion and evolution, provided that the mechanism(s) responsible for their production is known. However, to-date there is no general consensus on the origin of QPOs. An attractive mechanism for QPO production is disk tearing in tilted accretion flows, in which the disk tears apart leading to the formation of independently precessing sub-disks, the emission from which is expected to be quasi-periodic.

In this talk I discuss the discovery of high and low frequency QPO signatures in a general relativistic magnetohydrodynamic simulation of a highly tilted (65 degrees), geometrically thin accretion disk. We find that the Lense-Thirring torque from the rapidly spinning 10 solar mass black hole causes several sub-disks to tear off within ~10-20 gravitational radii. Tearing occurs in cycles on timescales of seconds. During each tearing cycle the inner sub-disk precesses for 1-5 periods before it falls into the black hole. We find a precession frequency of ~3Hz, consistent with observed low-frequency QPOs. In addition, we find a that a high frequency QPO (HFQPO) is produced by the radial epicyclic oscillations of a dense ring of gas at the tearing radius, which strongly suggests a corresponding modulation of the X-ray lightcurve and may thus explain some of the observed HFQPOs.