Excitation of Trapped Waves in Simulations of Tilted Black Hole Accretion Disks with Magnetorotational Turbulence

TL;DR

This study uses general relativistic magnetohydrodynamic simulations to analyze how disk tilt in black hole accretion flows excites inertial waves, revealing distinct variability patterns and composite oscillations.

Contribution

It demonstrates that disk tilt can excite inertial waves in accretion disks, providing detailed analysis of their structure and variability in relativistic simulations.

Findings

Tilted disks show excess inertial variability below epicyclic frequency.

Power at 118 Hz is due to a combination of dense clumps and inertial waves.

Disk tilt may be a key mechanism for inertial wave excitation.

Abstract

We analyze the time dependence of fluid variables in general relativistic, magnetohydrodynamic simulations of accretion flows onto a black hole with dimensionless spin parameter a/M=0.9. We consider both the case where the angular momentum of the accretion material is aligned with the black hole spin axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted flow). In comparison to the untilted simulation, the tilted simulation exhibits a clear excess of inertial variability, that is, variability at frequencies below the local radial epicyclic frequency. We further study the radial structure of this inertial-like power by focusing on a radially extended band at 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables. The three dimensional density structure at this frequency suggests that the power is a composite oscillation whose dominant components are an over dense clump corotating with the background flow, a low order inertial wave, and a low order inertial-acoustic wave. Our results provide preliminary confirmation of earlier suggestions that disk tilt can be an important excitation mechanism for inertial waves.