# Constraining black hole spins with low-frequency quasi-periodic oscillations in soft states

## TL;DR

This study uses low-frequency QPOs observed in soft states of black hole X-ray binaries to estimate black hole spins, providing constraints on spin values through the relativistic precession model.

## Contribution

It applies the relativistic precession model to low-frequency QPOs in soft states to constrain black hole spins, a novel approach in this context.

## Key findings

- Lower limits on black hole spins for all sources
- Upper limits on spins for five sources
- QPOs detected in soft states consistent with the RPM

## Abstract

Black hole X-ray transients show a variety of state transitions during their outburst phases, characterized by changes in their spectral and timing properties. In particular, power density spectra (PDS) show quasi periodic oscillations (QPOs) that can be related to the accretion regime of the source. We looked for type-C QPOs in the disc-dominated state (i.e. the high soft state) and in the ultra-luminous state in the RXTE archival data of 12 transient black hole X-ray binaries known to show QPOs during their outbursts. We detected 6 significant QPOs in the soft state that can be classified as type-C QPOs. Under the assumption that the accretion disc in disc-dominated states extends down or close to the innermost stable circular orbit (ISCO) and that type-C QPOs would arise at the inner edge of the accretion flow, we use the relativistic precession model (RPM) to place constraints on the black hole spin. We were able to place lower limits on the spin value for all the 12 sources of our sample while we could place also an upper limit on the spin for 5 sources.