Variability from Nonaxisymmetric Fluctuations Interacting with Standing Shocks in Tilted Black Hole Accretion Disks

TL;DR

This study uses 3D general relativistic magnetohydrodynamical simulations to analyze variability in tilted and untilted black hole accretion disks, highlighting the role of transient clumps, spiral waves, and standing shocks in variability amplification.

Contribution

It reveals that nonaxisymmetric clumps and spiral-shock interactions significantly influence variability, especially in tilted flows with standing shocks, providing new insights into accretion flow dynamics.

Findings

Tilted flows show greater variability at specific frequencies.

Transient clumps orbit on Keplerian trajectories regardless of tilt.

Standing shocks amplify variability caused by spiral waves.

Abstract

We study the spatial and temporal behavior of fluid in fully three-dimensional, general relativistic, magnetohydrodynamical simulations of both tilted and untilted black hole accretion flows. We uncover characteristically greater variability in tilted simulations at frequencies similar to those predicted by the formalism of trapped modes, but ultimately conclude that its spatial structure is inconsistent with a modal interpretation. We find instead that previously identified, transient, over-dense clumps orbiting on roughly Keplerian trajectories appear generically in our global simulations, independent of tilt. Associated with these fluctuations are acoustic spiral waves interior to the orbits of the clumps. We show that the two nonaxisymmetric standing shock structures that exist in the inner regions of these tilted flows effectively amplify the variability caused by these spiral waves to markedly higher levels than in untilted flows, which lack standing shocks. Our identification of clumps, spirals, and spiral-shock interactions in these fully general relativistic, magnetohydrodynamical simulations suggests that these features may be important dynamical elements in models which incorporate tilt as a way to explain the observed variability in black hole accretion flows.