An Attempt to Describe Frequency Correlations among kHz QPOs and HBOs by Two-Armed Nearly Vertical Oscillations

TL;DR

This study models two-armed vertical p-mode oscillations in magnetized accretion disks to explain observed frequency correlations among kHz QPOs and HBOs in low-mass X-ray binaries, suggesting disk thickness and magnetic field variations influence these phenomena.

Contribution

It introduces a new oscillation model based on two-armed vertical p-modes to explain frequency correlations in LMXBs, linking disk properties to observed QPOs and HBOs.

Findings

Frequency correlation matches observations with reasonable parameters.

Inner disk regions are likely thinned by hot coronae.

HBOs may be first overtone vertical oscillations.

Abstract

We examine whether the two-armed ($m=2$) vertical p-mode oscillations trapped in the innermost region of magnetized accretion disks with finite disk thickness can describe kHz QPOs and HBOs in LMXBs. First, we derive the frequency-frequency correlation of the two basic oscillations (both are fundamental modes in the vertical direction, but one is the fundamental and the other the first overtone in the radial direction), and compare it with the observed frequency correlation of twin kHz QPOs. Results show that the calculated frequency correlation can well describe observed correlation with reasonable values of parameters. Second, we examine whether the observed frequency correlation between kHz QPOs and HBO can be described by regarding HBO as the first overtone oscillation in the vertical direction (and the fundamental in the radial direction). The results suggest that i) innermost parts of disks on the horizontal branch are strongly diminished in their vertical thickness (presumably by hot coronae) and ii) the branch is roughly a sequence of variations of magnetic fields or disk temperature.