High frequency oscillations in outbursts of Kerr-metric slim disks

TL;DR

This paper numerically studies thermally unstable Kerr-metric slim disks, revealing trapped p-mode oscillations and overtones in limit-cycle outbursts, with implications for black hole spin measurements via high-frequency QPOs.

Contribution

It introduces a new model using an evolutionary viscous stress equation and identifies trapped p-mode oscillations with overtones in slim disks.

Findings

Oscillations occur near the inner boundary and propagate outward.

Fundamental harmonic frequency matches the maximum epicyclic frequency.

Frequency ratios of overtones are integer series.

Abstract

We numerically investigate the thermally unstable accretion disks around black holes. We adopt an evolutionary viscous stress equation to replace the standard alpha-prescription based on the results of two MHD simulations. We find a kind of interesting oscillations on some running models in limit-cycle outburst state. The oscillations arise near the inner boundary and propagate radially outwards. We deem that they are the trapped $p$-mode oscillations excited by sonic-point instability. We directly integrate the local radiation cooling fluxes to construct the mimic bolometric light-curve. We find a series of overtones beside the fundamental harmonic on the power spectra of mimic light-curves. The frequency of the fundamental harmonic is very close to the maximum epicyclic frequency of the disk and the frequency ratio of the fundamental harmonic and overtones is a regular integer series. We suggest that the code for ray-tracing calculation must be time-dependent in virtual observation and point out the robustness of the black hole spin measurement with high frequency QPOs.