Title: Constraining black hole spins with low-frequency quasi-periodic oscillations in soft states,
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.