Inertial-Acoustic Oscillations of Black-Hole Accretion Discs with Large-Scale Poloidal Magnetic Fields

TL;DR

This paper investigates how large-scale poloidal magnetic fields influence inertial-acoustic oscillations in black-hole accretion disks, potentially explaining observed high-frequency quasi-periodic oscillations and their characteristics.

Contribution

It demonstrates that large-scale magnetic fields can enhance mode growth rates and adjust frequencies, improving the match with observational data.

Findings

Magnetic fields increase the growth rate of p-modes.

Magnetic fields reduce the mode frequencies.

Enhanced instability correlates with observed HFQPOs.

Abstract

We study the effect of large-scale magnetic fields on the non-axisymmetric inertial-acoustic modes (also called p-modes) trapped in the innermost regions of accretion discs around black holes (BHs). These global modes could provide an explanation for the high-frequency quasi-periodic oscillations (HFQPOs) observed in BH X-ray binaries. There may be observational evidence for the presence of such large-scale magnetic fields in the disks since episodic jets are observed in the same spectral state when HFQPOs are detected. We find that a large-scale poloidal magnetic field can enhance the corotational instability and increase the growth rate of the purely hydrodynamic overstable p-modes. In addition, we show that the frequencies of these overstable p-modes could be further reduced by such magnetic fields, making them agree better with observations.