Low frequency QPO spectra and Lense-Thirring precession

TL;DR

This paper presents a model explaining low frequency QPOs in black hole binaries through Lense-Thirring precession of an extended hot inner flow, successfully matching observed frequencies and spectra.

Contribution

It introduces a radially extended precession model that accounts for constant maximum QPO frequency around 10Hz, independent of black hole spin, aligning with observations.

Findings

Precession of extended hot inner flow explains QPO frequencies.

Model predicts maximum QPO frequency around 10Hz regardless of spin.

The model aligns with observed spectral and timing properties.

Abstract

We show that the low frequency QPO seen in the power density spectra of black hole binaries (and neutron stars) can be explained by Lense-Thirring precession. This has been proposed many times in the past, and simple, single radius models can qualitatively match the observed increase in QPO frequency by decreasing a characteristic radius, as predicted by the truncated disc models. However, this also predicts that the frequency is strongly dependent on spin, and gives a maximum frequency at the last stable orbit which is generally much higher than the remarkably constant maximum frequency at ~10Hz observed in all black hole binaries. The key aspect of our model which makes it match these observations is that the precession is of a radially extended region of the hot inner flow. The outer radius is set by the truncation radius of the disc as above, but the inner radius lies well outside of the last stable orbit at the point where numerical simulations show that the density drops off sharply for a misaligned flow. Physically motivated analytic estimates for this inner radius show that it increases with a_*, decreasing the expected frequency in a way which almost completely cancels the expected increase with spin. This ties the maximum predicted frequency to around 10Hz irrespective of a_*, as observed. This is the first QPO model which explains both frequencies and spectrum in the context of a well established geometry for the accretion flow.