High-Frequency QPOs and Overstable Oscillations of Black-Hole Accretion Disks

TL;DR

This paper reviews the physical mechanisms behind high-frequency QPOs in black-hole accretion disks, emphasizing the role of global oscillation modes and relativistic effects in explaining observed frequencies and ratios.

Contribution

It introduces a semi-analytic model based on disk oscillation modes and relativistic effects that explains HFQPO frequencies and their characteristic ratios.

Findings

Computed mode frequencies match observed HFQPOs

The 2:3 frequency ratio naturally arises from the model

Global spiral density modes are robust with magnetic fields and turbulence

Abstract

The physical origin of high-frequency QPOs (HFQPOs) in black-hole X-ray binaries remains an enigma despite many years of detailed observational studies. Although there exists a number of models for HFQPOs, many of these are simply "notions" or "concepts" without actual calculation derived from fluid or disk physics. Future progress requires a combination of numerical simulations and semi-analytic studies to extract physical insights. We review recent works on global oscillation modes in black-hole accretion disks, and explain how, with the help of general relativistic effects, the energy stored in the disk differential rotation can be pumped into global spiral density modes in the disk, making these modes grow to large amplitudes under certain conditions ("corotational instability"). These modes are robust in the presence of disk magnetic fields and turbulence. The computed oscillation mode frequencies are largely consistent with the observed values for HFQPOs in BH X-ray binaries. The approximate 2:3 frequency ratio is also expected from this model. The connection of HFQPOs with other disk properties (such as production of episodic jets) is also discussed.