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

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 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 ultraluminous state in the RXTE archival data of 12 transient black hole X-ray binaries known to show QPOs during their outbursts. We detected six 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 and that type-C QPOs would arise at the inner edge of the accretion flow, we use the relativistic precession model 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.