A Resonantly-Excited Disk-Oscillation Model of High-Frequency QPOs of Microquasars

TL;DR

This paper proposes a resonant disk-oscillation model involving magnetic fields and deformations to explain twin high-frequency QPOs in microquasars, linking observed frequencies to disk parameters and magnetic field strengths.

Contribution

It introduces a novel resonant interaction mechanism between vertical p-modes and g-modes in magnetized, deformed disks to explain high-frequency QPOs.

Findings

High spins can be explained with weak poloidal magnetic fields.

The 3:2 frequency ratio occurs by chance, not excitation.

Disk parameters influence QPO frequencies and source spin estimates.

Abstract

A possible model of twin high-frequency QPOs (HF QPOs) of microquasars is examined. The disk is assumed to have global magnetic fields and to be deformed with a two-armed pattern. In this deformed disk, set of a two-armed ($m=2$) vertical p-mode oscillation and an axisymmetric ($m=0$) g-mode oscillation are considered. They resonantly interact through the disk deformation when their frequencies are the same. This resonant interaction amplifies the set of the above oscillations in the case where these two oscillations have wave energies of opposite signs. These oscillations are assumed to be excited most efficiently in the case where the radial group velocities of these two waves vanish at the same place. The above set of oscillations is not unique, depending on the node number, $n$, of oscillations in the vertical direction. We consider that the basic two sets of oscillations correspond to the twin QPOs. The frequencies of these oscillations depend on disk parameters such as strength of magnetic fields. For observational mass ranges of GRS 1915+105, GRO J1655-40, XTE J1550-564, and H1743-322, spins of these sources are estimated. High spins of these sources can be described if the disks have weak poloidal magnetic fields as well as toroidal magnetic fields of moderate strength. In this model the 3 : 2 frequency ratio of high-frequency QPOs is not related to their excitation, but occurs by chance.